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WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility for the William Herschel Telescope, will see first light in late 2022. WEAVE comprises a new 2-degree field-of-view prime-focus corrector system, a nearly 1000-multiplex fibre positioner, 20 individually deployable 'mini' integral field units (IFUs), and a single large IFU. These fibre systems feed a dual-beam spectrograph covering the wavelength range 366$-$959\,nm at $R\sim5000$, or two shorter ranges at $R\sim20\,000$. After summarising the design and implementation of WEAVE and its data systems, we present the organisation, science drivers and design of a five- to seven-year programme of eight individual surveys to: (i) study our Galaxy's origins by completing Gaia's phase-space information, providing metallicities to its limiting magnitude for $\sim$3 million stars and detailed abundances for $\sim1.5$ million brighter field and open-cluster stars; (ii) survey $\sim0.4$ million Galactic-plane OBA stars, young stellar objects and nearby gas to understand the evolution of young stars and their environments; (iii) perform an extensive spectral survey of white dwarfs; (iv) survey $\sim400$ neutral-hydrogen-selected galaxies with the IFUs; (v) study properties and kinematics of stellar populations and ionised gas in $z1$ million spectra of LOFAR-selected radio sources; (viii) trace structures using intergalactic/circumgalactic gas at $z>2$. Finally, we describe the WEAVE Operational Rehearsals using the WEAVE Simulator., 41 pages, 27 figures, accepted for publication by MNRAS

In humans, assembly of spliceosomal snRNPs (small nuclear ribonucleoproteins) begins in the cytoplasm where the multi-protein SMN (survival of motor neuron) complex mediates the formation of a seven-membered ring of Sm proteins on to a conserved site of the snRNA (small nuclear RNA). The SMN complex contains the SMN protein Gemin2 and several additional Gemins that participate in snRNP biosynthesis. SMN was first identified as the product of a gene found to be deleted or mutated in patients with the neurodegenerative disease SMA (spinal muscular atrophy), the leading genetic cause of infant mortality. In the present study, we report the solution structure of Gemin2 bound to the Gemin2-binding domain of SMN determined by NMR spectroscopy. This complex reveals the structure of Gemin2, how Gemin2 binds to SMN and the roles of conserved SMN residues near the binding interface. Surprisingly, several conserved SMN residues, including the sites of two SMA patient mutations, are not required for binding to Gemin2. Instead, they form a conserved SMN/Gemin2 surface that may be functionally important for snRNP assembly. The SMN–Gemin2 structure explains how Gemin2 is stabilized by SMN and establishes a framework for structure–function studies to investigate snRNP biogenesis as well as biological processes involving Gemin2 that do not involve snRNP assembly.

We present K-band integral field observations of the circumstellar envelope of the evolved star OH 231.8+4.2. Spatial and spectral information were simultaneously acquired using the Sinfoni integral field unit, with adaptive optics, on the Very Large Telescope. The observations reveal the discovery of H2 emission (1) around the centre of the nebula and (2) located in clumps along the Western side of the Northern lobe, presumably associated with the strong shocks that stimulate the previously reported H-alpha emission at the same location. An observed H2 1-0/2-1S(1) line ratio of 8.3+/-1.9 was calculated for the central field, a value consistent with shock excitation.

Sparse sampling in biomolecular multidimensional NMR offers increased acquisition speed and resolution and, if appropriate conditions are met, an increase in sensitivity. Sparse sampling of indirectly detected time domains combined with the direct truly multidimensional Fourier transform has elicited particular attention because of the ability to generate a final spectrum amenable to traditional analysis techniques. A number of sparse sampling schemes have been described including radial sampling, random sampling, concentric sampling and variations thereof. A fundamental feature of these sampling schemes is that the resulting time domain data array is not amenable to traditional Fourier transform based processing and phasing correction techniques. In addition, radial sampling approaches offer a number of advantages and capabilities that are also not accessible using standard NMR processing techniques. These include sensitivity enhancement, sub-matrix processing and determination of minimal sets of sampling angles. Here we describe a new software package (Al NMR) that enables these capabilities in the context of a general NMR data processing environment.

Radial sampling in multidimensional NMR experiments offers greatly decreased acquisition times while also providing an avenue for increased sensitivity. Digital resolution remains a concern and depends strongly upon the extent of sampling of individual radial angles. Truncated time domain data leads to spurious peaks (artifacts) upon FT and 2D FT. Linear prediction is commonly employed to improve resolution in Cartesian sampled NMR experiments. Here, we adapt the linear prediction method to radial sampling. Significantly more accurate estimates of linear prediction coefficients are obtained by combining quadrature frequency components from the multiple angle spectra. This approach results in significant improvement in both resolution and removal of spurious peaks as compared to traditional linear prediction methods applied to radial sampled data. The ‘averaging linear prediction’ (ALP) method is demonstrated as a general tool for resolution improvement in multidimensional radial sampled experiments.

The Cartesian sampled three-dimensional HNCO experiment is inherently limited in time resolution and sensitivity for the real time measurement of protein hydrogen exchange. This is largely overcome by use of the radial HNCO experiment that employs the use of optimized sampling angles. The significant practical limitation presented by use of three-dimensional data is the large data storage and processing requirements necessary and is largely overcome by taking advantage of the inherent capabilities of the 2D-FT to process selective frequency space without artifact or limitation. Decomposition of angle spectra into positive and negative ridge components provides increased resolution and allows statistical averaging of intensity and therefore increased precision. Strategies for averaging ridge cross sections within and between angle spectra are developed to allow further statistical approaches for increasing the precision of measured hydrogen occupancy. Intensity artifacts potentially introduced by over-pulsing are effectively eliminated by use of the BEST approach.

Some of the earliest polarimetric measurements made in astronomy were concerned with the polarization of the interstellar medium resulting from dust grains aligned in the Galactic magnetic field. More than 50 years later, polarimetry continues to be an important diagnostic of field structure on size scales ranging from planetary to galactic. The use of both linear and circular polarimetry at optical and infrared wavelengths can provide additional insights into the nature of dust particles, their alignment in magnetic fields and the field topology. Given the science benefits that polarimetry offers it is perhaps surprising that the continued existence of polarimetric facilities on current and next generation large telescopes needs to be ensured.

Vasoactive intestinal peptide (VIP) mediates a broad range of biological responses by activating two related receptors, VIP receptor 1 and 2 (VIPR1 and VIPR2). Although the use of native VIP facilitates neuroprotection, clinical application of the hormone is limited due to VIP's rapid metabolism and inability to distinguish between VIPR1 and VIPR2 receptors. In addition, activation of both receptors by therapeutics may increase adverse secondary toxicities. Therefore, we developed metabolically stable and receptor-selective agonists for VIPR1 and VIPR2 to improve pharmacokinetic and pharmacodynamic therapeutic end points. Selective agonists were investigated for their abilities to protect mice against MPTP-induced neurodegeneration used to model Parkinson's disease (PD). Survival of tyrosine hydroxylase neurons in the substantia nigra was determined by stereological tests after MPTP intoxication in mice pretreated with either VIPR1 or VIPR2 agonist or after adoptive transfer of splenic cell populations from agonist-treated mice administered to MPTP-intoxicated animals. Treatment with VIPR2 agonist or splenocytes from agonist-treated mice resulted in increased neuronal sparing. Immunohistochemical tests showed that agonist-treated mice displayed reductions in microglial responses, with the most pronounced effects in VIPR2 agonist-treated, MPTP-intoxicated mice. In parallel studies, we observed reductions in proinflammatory cytokine release that included IL-17A, IL-6, and IFN-γ and increases in GM-CSF transcripts in CD4+T cells recovered from VIPR2 agonist-treated animals. Moreover, a phenotypic shift of effector to regulatory T cells was observed. These results support the use of VIPR2-selective agonists as neuroprotective agents for PD treatment.SIGNIFICANCE STATEMENTVasoactive intestinal peptide receptor 2 can elicit immune transformation in a model of Parkinson's disease (PD). Such immunomodulatory capabilities can lead to neuroprotection by attenuating microglial activation and by slowing degradation of neuronal cell bodies and termini in MPTP-intoxicated mice. The protective mechanism arises from altering a Th1/Th2 immune cytokine response into an anti-inflammatory and neuronal sparing profile. These results are directly applicable for the development of novel PD therapies.

Multi-colour imaging polarimetry of the galaxy NGC 5128 shows that polarization is present throughout the dust lane with patterns of orientation which are coherent on kpc scale-lengths. In the central regions of the dust lane, within ±O.7 kpc of the projected position of the galactic nucleus, the polarization is oriented parallel to the long axis of the lane and reaches levels up to 5 per cent. At the outer extremities of the the dust lane, higher levels of polarization are found with orientations perpendicular to the axis of the dust lane. In the intermediate regions, null points in the polarization are observed in addition to an unexpected reversal of polarization orientation across the dust lane. We are unable to account for all our observations - polarization maps and the wavelength dependence of polarization in the dust lane - without involving two competing polarizing mechanisms, namely, dichroic extinction and scattering by dust grains. The polarimetric properties ofNGC 5128 are very similar to those found in galaxies such as M104, IC 4329A and NGC 2146, and it appears that azimuthal magnetic fields, confined to the equatorial planes of galaxies with prominent dust lanes, are a common occurrence. The nature and origins of these magnetic fields are considered.

Dr Ilčiukienė’s monograph argues convincingly for placing music at centre of L2 language curriculum in the Lithuanian primary school in face of the fact that music has up to the present time been treated as an ancillary classroom activity in the L2 primary classroom in Lithuania. (This is also the case in most European L2 primary curricula such as in Britain and Germany where the musical activities are usually confined to the status of an optional extra and, at most, are regarded as an effective form of consolidation of material covered in any particular language input unit in a course book.) Ilčiukienė’s argumentation demonstrates not only that music motivates learners by providing a more relaxing and enjoyable classroom atmosphere (positive affect) and that the prosodic features music can provide an excellent basis for acquiring English intonation but also that music can even facilitate the acquisition of syntax, idiom and lexis. For these reasons alone, music should acquire a new status in L2 primary courses.

Comprehensive application of solution NMR spectroscopy to studies of macromolecules remains fundamentally limited by the molecular rotational correlation time. For proteins, molecules larger than 30 kDa require complex experimental methods, such as TROSY in conjunction with isotopic labeling schemes that are often expensive and generally reduce the potential information available. We have developed the reverse micelle encapsulation strategy as an alternative approach. Encapsulation of proteins within the protective nano-scale water pool of a reverse micelle dissolved in ultra-low viscosity nonpolar solvents overcomes the slow tumbling problem presented by large proteins. Here, we characterize the contributions from the various components of the protein-containing reverse micelle system to the rotational correlation time of the encapsulated protein. Importantly, we demonstrate that the protein encapsulated in the reverse micelle maintains a hydration shell comparable in size to that seen in bulk solution. Using moderate pressures, encapsulation in ultra-low viscosity propane or ethane can be used to magnify this advantage. We show that encapsulation in liquid ethane can be used to reduce the tumbling time of the 43 kDa maltose binding protein from ~23 ns to ~10 ns. These conditions enable, for example, acquisition of TOCSY-type data resolved on the adjacent amide NH for the 42 kDa encapsulated maltose binding protein dissolved in liquid ethane, which is typically impossible for proteins of such size without use of extensive deuteration or the TROSY effect.

An optical polarization map of the GGD 30 nebulosity shows that, although it shares many of the morphological characteristics of classical cometary nebulae such as NGC 2261/R.Mon, it differs in that the star-like knot at its head is not the illluminating and exciting source as previously assumed. The knot is reflection nebulosity and the true source, completely hidden from direct view at optical wave-lengths, is located some 6 arcsec east of the knot

Members of the leiognathid subfamily Gazzinae, comprising approximately two-thirds of ponyfish species, are sexually dimorphic with regard to features of the light organ system (LOS). In Gazzinae, the circumesophageal light organ (LO) of males is enlarged and varies in shape compared with similarly sized conspecific females. In association with male species-specific translucent external patches on the head and flank, these sexually dimorphic LO features are hypothesized to be correlated with species-specific luminescence displays. Anatomical differences in LO shape, volume, and orientation, and its association with the gas bladder and other internal structures that function in light emission, are compared to observations of luminescence displays for every major lineage within Leiognathidae. We reconstruct the character evolution of both internal and external morphological features of the LOS to investigate the evolution of LO sexual dimorphism and morphology. Both internal and external sexual dimorphism in the ponyfish LOs were recovered as most likely to have evolved in the common ancestor of Leiognathidae, and likelihood-based correlation analyses indicate that the evolution of internal and external dimorphism in males is statistically correlated. Magnetic resonance imaging technology was applied to examine the unique internal LOs of ponyfishes in situ, which provides a new metric (LO index) for comparison of LO structure across lineages. J. Morphol. 2011. © 2011 Wiley-Liss, Inc.

NGC 4151 shows structure in the levels and pattern of polarization on all size-scales. On the large scale the polarization pattern tends to follow the spiral structure of the galaxy which can be traced to within ∼ 20 arcsec of the nucleus in the I waveband. In the V and Hα wavebands there is a jet-like structure of polarized (scattered) light at position angle 230° which coincides with the ENLR while the nuclear polarization is orthogonal to the arcsec-scale radio jets at position angles of ∼ 80°. The results show that there are polarization features, arising from the scattering of light from the central nucleus, allied to both the radio and ENLR collimation axes

Sparse sampling offers tremendous potential for overcoming the time limitations imposed by traditional Cartesian sampling of indirectly detected dimensions of multidimensional NMR data. However, in many instances sensitivity rather than time remains of foremost importance when collecting data on protein samples. Here we explore how to optimize the collection of radial sampled multidimensional NMR data to achieve maximal signal-to-noise. A method is presented that exploits a rigorous definition of the minimal set of radial sampling angles required to resolve all peaks of interest in combination with a fundamental statistical property of radial sampled data. The approach appears general and can achieve a substantial sensitivity advantage over Cartesian sampling for the same total data acquisition time. Termed Sensitivity Enhanced n-Dimensional or SEnD NMR, the method involves three basic steps. First, data collection is optimized using routines to determine a minimal set of radial sampling angles required to resolve frequencies in the radially sampled chemical shift evolution dimensions. Second, appropriate combinations of experimental parameters (transients and increments) are defined by simple statistical considerations in order to optimize signal-to-noise in single angle frequency domain spectra. Finally, the data is processed with a direct multidimensional Fourier transform and a statistical artifact and noise removal step is employed.

Based on an international workshop (Gothenburg, 14–16 May 2008), this review article aims to combine interdisciplinary knowledge from coastal and open ocean research on iron biogeochemistry. The major scientific findings of the past decade are structured into sections on natural and artificial iron fertilization, iron inputs into coastal and estuarine systems, colloidal iron and organic matter, and biological processes. Potential effects of global climate change, particularly ocean acidification, on iron biogeochemistry are discussed. The findings are synthesized into recommendations for future research areas.

Imaging polarimetry of the reflection nebula NGC 7023 in BVRI wavebands show there are localized regions within the nebula where the levels of polarization at R and I are significantly reduced relative to those at B and V. In these regions there appears to be broadband R-NIR emission which dilutes the scattered radiation to create the reduced levels of polarization. We believe that this emission is ERE and arises from one component of the grains which scatter the central starlight to produce the reflection nebula. In the R and I wavebands this emission is ∼15-20 per cent of the total light coming from the selected regions of the nebula

Imaging polarimetry of the β Pic disc shows that it is reflection nebulosity solely illuminated by the central star. The level of polarization in both segments of the disc is 17 ± 3 per cent (R waveband) and is relatively constant in the distance range 15-30 arcsec from the star. It is found that the β Pic disc shows up in polarized light when we treat the light surrounding the bright star as simple reflection nebulosity. This procedure does not require complicated background subtractions from reference stars and should provide a simple and fast method of locating planetary and protoplanetary discs surrounding other stars

A simple model of a polarized source is used to produce intensity isophotes and linear polarization maps simulating the scattering of light in bipolar nebulae. The results indicate that features such as polarization null points and bands of parallel vectors can be explained in terms of a polarized source without recourse to aligned grains. The particular case of the reflection nebula NGC 2261 is considered in detail and it is shown how the model can account for the observed variability in the polarization of this object

Sparse sampling offers tremendous potential for overcoming the time limitations imposed by traditional Cartesian sampling of indirectly detected dimensions of multidimensional NMR data. Unfortunately, several otherwise appealing implementations are accompanied by spectral artifacts that have the potential to contaminate the spectrum with false peak intensity. In radial sampling of linked time evolution periods, the artifacts are easily identified and removed from the spectrum if a sufficient set of radial sampling angles is employed. Robust implementation of the radial sampling approach therefore requires optimization of the set of radial sampling angles collected. Here we describe several methods for such optimization. The approaches described take advantage of various aspects of the general simultaneous multidimensional Fourier transform in the analysis of multidimensional NMR data. Radially sampled data are primarily contaminated by ridges extending from authentic peaks. Numerical methods are described that definitively identify artifactual intensity and the optimal set of sampling angles necessary to eliminate it under a variety of scenarios. The algorithms are tested with both simulated and experimentally obtained triple resonance data.

Une carte de la polarisation optique du globule cometaire CG 30 montre qu'il s'agit d'une petite aire de nebuleuse reflechissante illuminee de l'interieur du globule sombre par une source excitatrice identifiee avec la source IRAS 08076-3356. La structure dans les donnees de polarisation suggere que la nebuleuse optique est formee par l'illumination des parois d'une cavite creusee dans le nuage par l'activite stellaire

The recent re-introduction of the two-dimensional Fourier transformation (2D-FT) has allows for the transformation of arbitrarily sampled time domain signals. In this respect, radial sampling, where two incremented time dimensions ( t 1 and t 2 ) are sampled such that t 1 = τ cos α and t 2 = τ sin α , is especially appealing because of the relatively small leakage artifacts that occur upon Fourier transformation. Unfortunately radially sampled time domain data results in a fundamental artifact in the frequency domain manifested as a ridge of intensity extending through the peak positions perpendicular to +/− the radial sampling angle. Successful removal of the ridge artifacts using existing algorithms requires absorptive line shapes. Here we present two procedures for retrospective phase correction of arbitrarily sampled data.

We have carried out JHK polarimetric observations of eleven dusty young stars, by using the polarimeter module IRPOL2 with the near-IR camera UIST on the 3.8-m United Kingdom Infrared Telescope (UKIRT). Our sample targeted systems for which UKIRT-resolvable discs had been predicted by model fits to their spectral energy distributions. Our observations have confirmed the presence of extended polarized emission around TW Hya and around HD 169142. HD 150193 and HD 142666 show the largest polarization values among our sample, but no extended structure was resolved. By combining our observations with HST coronographic data from the literature, we derive the J- and H-band intrinsic polarization radial dependences of TW Hya's disc. We find the disc's polarizing efficiency is higher at H than at J, and we confirm that the J- and H-band percentage polarizations are reasonably constant with radius in the region between 0.9 and 1.3 arcseconds from the star. We find that the objects for which we have detected extended polarizations are those for which previous modelling has suggested the presence of flared discs, which are predicted to be brighter than flat discs and thus would be easier to detect polarimetrically., Comment: Accepted by MNRAS

The influences of feedback and ascending and descending trial sequences on the ability of 135 college-aged subjects to detect phenyl ethyl alcohol odorant concentrations ranging from 10(-9) to 10(-5.5) v/v were examined in a two-alternative forced-choice test paradigm. At the highest concentrations, ascending trial sequences produced better performance than descending trial sequences; the reverse was true at the lowest concentrations. There was a tendency for feedback to improve performance marginally at the lowest two odorant concentrations presented. In the region associated with a traditional detection threshold calculation (i.e. at the 75% performance point in a two-choice detection task), no influences of feedback or direction of trial sequence were apparent. These data indicate that the effects of explicit feedback and trial sequence direction depend upon the segment of the peri-threshold stimulus concentration continuum evaluated.

Comet Austin (1989c 1 ) was observed during 1990 May and June when its phase angle changed from 26° to 8°. The degree of linear polarization was seen to change from positive to negative polarizations at a phase angle of 20.°5, with a rate of change of 0.19 per cent per degree in the R waveband. This phase dependence of polarization is similar to that found for other comets, except that the dust grains giving rise to the polarization have a higher albedo than those found in other comets at similar distances from the Sun. No unusual activity in the form of jets or shells is seen in the polarization images

We have obtained new near-infrared images of both GG Tau and UY Aur circumbinary disks with the polarimetric modes of NICMOS aboard the Hubble Space Telescope. The 1 micron intensity map of GG Tau reveals a more complete elliptical shape than in previous ground-based images. Its eastern and western sides are definitely asymmetric. Our image strongly supports the ring geometry proposed by Guilloteau, Dutrey & Simon (1999) on the basis of their millimetre interferometry images: a geometrically thick and sharply edged ring surrounding an empty gap around the binary. Around UY Aur, we identify structures that are in excellent agreement with the optical images of M��nard et al. (1999), which confirms that the inclination of the system to the line-of-sight is about 60 degrees. We also find tentative new structures closer to the stars., 6 pages, including 6 figures, proceedings of the Euroconference on "Disk, planetesimal and planets", Garzon, Eiroa, de Winter and Mahoney eds., ASP conf. series

Strong infrared circular polarization resulting from dust scattering in reflection nebulae in the Orion OMC-1 star-formation region has been observed. Circular polarization at shorter wavelengths might have been important in inducing chiral asymmetry in interstellar organic molecules that could be subsequently delivered to the early Earth by comets, interplanetary dust particles, or meteors. This could account for the excess of L-amino acids found in the Murchison meteorite and could explain the origin of the homochirality of biological molecules.

The asymmetries in the brightness structure and levels of polarization between the two sides of the β Pic disc are discussed. The possibility of these differences being caused by a planet, or planets, within the circumstellar disc is investigated.

Museum fish collections possess a wealth of anatomical and morphological data that are essential for documenting and understanding biodiversity. Obtaining access to specimens for research, however, is not always practical and frequently conflicts with the need to maintain the physical integrity of specimens and the collection as a whole. Non-invasive three-dimensional (3D) digital imaging therefore serves a critical role in facilitating the digitization of these specimens for anatomical and morphological analysis as well as facilitating an efficient method for online storage and sharing of this imaging data. Here we describe the development of the Digital Fish Library (DFL, http://www.digitalfishlibrary.org), an online digital archive of high-resolution, high-contrast, magnetic resonance imaging (MRI) scans of the soft tissue anatomy of an array of fishes preserved in the Marine Vertebrate Collection of Scripps Institution of Oceanography. We have imaged and uploaded MRI data for over 300 marine and freshwater species, developed a data archival and retrieval system with a web-based image analysis and visualization tool, and integrated these into the public DFL website to disseminate data and associated metadata freely over the web. We show that MRI is a rapid and powerful method for accurately depicting the in-situ soft-tissue anatomy of preserved fishes in sufficient detail for large-scale comparative digital morphology. However these 3D volumetric data require a sophisticated computational and archival infrastructure in order to be broadly accessible to researchers and educators.

To determine the effects of exposure of endothelial cells to therapeutic ultrasound on leucocyte adhesion to these cells, confluent cultures of bovine aortic endothelial cells in tissue culture flasks and on collagen coated coverslips were exposed to a maximum of 1.6 W/cm2 1 MHz continuous ultrasound for 15 min, then incubated with neutrophil-rich suspensions containing a known number of viable human leucocytes for intervals from 1 to 240 min. After incubation, nonadherent cells were decanted, the number of leucocytes in the eluted suspension determined and the percentage of adhered cells calculated. Endothelial cell viability was not significantly reduced by ultrasound or by incubation with leucocytes. Whereas untreated endothelial cells were flattened and had surface microvilli, those exposed to ultrasound were plump, rounded and more widely separated along their borders. The percentage of leucocytes that adhered to sonicated endothelium was significantly higher, and this proportion increased with the duration of incubation. Thus, exposure of endothelial cells to ultrasound in vitro can cause intercellular separation and increase the adhesion to leucocytes. If such changes were also to occur in vivo, therapeutic ultrasound would potentiate the inflammatory response.

The most common technique for bone mineral densitometry is Dual Photon Absorptiometry with a dedicated scanning device. In principle, a gamma camera could be adapted to perform the same function, but little work along these lines has been reported in the literature. Tests made using a standard gamma camera with a 153Gd source at the focus of a magnifying collimator show that satisfactory results can be obtained on water/aluminium phantoms. A calibration is used to help ensure long term precision and stability. Measurements made using the same equipment but 241Am/99mTc sources gave poor results.

Etude de la polarisation du complexe Haro 6-5. On a identifie les sources d'illumination et on suggere une interpretation geometrique du systeme. Dans la bande R,w le jet est polarise lineairement

Previous work has shown that the lethal effects of glucocorticoids on the human lymphoblastoid cell line, CEM-C7, are antagonized by the simultaneous presence of 1-beta-D-arabinofuranosylcytosine (Ara-C). A possible cell cycle mechanism prompted further studies using flow microfluorimetry. We now report that (1) Ara-C (10-100 nM) blocks cells in S-phase and (2) the block is reversible after the drug is removed. A second treatment protocol, in which glucocorticoid is added to cells recovering from the effects of 24 h exposure to Ara-C, results in a clear synergism between the 2 drugs. This synergism is observed over a range of concentrations (5-100 nM), but is most significant at low doses, where inhibition of cell growth by Ara-C occurs but cell killing is minimal. Prior treatment with Ara-C increases the number of cells killed in the presence of steroid during the period 12-24 h after removal of the S-phase block. Combinations of Ara-C and steroid can thus be either synergistic or antagonistic, depending on the drug scheduling.