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A non-Ohmic current that grows exponentially with the square root of applied electric field is well known from thermionic field emission (the Schottky effect), electrolytes (the second Wien effect) and semiconductors (the Poole–Frenkel effect). It is a universal signature of the attractive Coulomb force between positive and negative electrical charges, which is revealed as the charges are driven in opposite directions by the force of an applied electric field. Here we apply thermal quenches to spin ice to prepare metastable populations of bound pairs of positive and negative emergent magnetic monopoles at millikelvin temperatures. We find that the application of a magnetic field results in a universal exponential-root field growth of magnetic current, thus confirming the microscopic Coulomb force between the magnetic monopole quasiparticles and establishing a magnetic analogue of the Poole–Frenkel effect. At temperatures above 300 mK, gradual restoration of kinetic monopole equilibria causes the non-Ohmic current to smoothly evolve into the high-field Wien effect for magnetic monopoles, as confirmed by comparison to a recent and rigorous theory of the Wien effect in spin ice. Our results extend the universality of the exponential-root field form into magnetism and illustrate the power of emergent particle kinetics to describe far-from-equilibrium response in complex systems.

We present the results of a combined neutron scattering and magnetization study of the pseudo-one-dimensional magnetic compounds [ (CH3)3NH] Co1-xNixCl3.2 H2O, abbreviated as CoNiTAC. For Co rich samples, we determined the 3d magnetic structure to be canted antiferromagnetic having the magnetic space group Pnm'a'. We performed Monte-Carlo simulations and could reproduce well the measured temperature dependence of the sublattice magnetization assuming an Ising type, spatially strongly anisotropic interaction. The exchange integral along the chains (b-direction) is at least 100 times larger than the average exchange in the a-c-plane. Despite this strong one dimensional character of the exchange interactions the critical exponent β was found to be very close to the theoretical value for a 3d Ising system (0.31). We did not discover any anomaly of the long range order component for the mixed crystals, for which a spin glass phase had been proposed by other authors on the basis of a decay of the TRM on macroscopic time scales. Instead, our polarized neutron diffraction and magnetization measurements reveal that the low temperature dynamics is connected to domain relaxation processes and not to a spin glass transition.

Purpose The purpose of this study is to summarize new data on etiology and clinical features of Klinefelter syndrome in order to derive research priorities.Methods This study was conducted using critical reviews of selective topics, emphasizing less well-recognized clinical findings.Results And Conclusions The phenotype of the prototypic 47,XXY case is well recognized: seminiferous tubule dysgenesis and androgen deficiency. Less well appreciated is the varied expressivity of 47,XXY Klinefelter syndrome, in particular neurological/cognitive perturbations like language and behavioral problems. Effective therapies are available. Reproductive technologies allow 47,XXY men to sire offspring through intracytoplasmic sperm injection (ICSI); however, genetic counseling is complex and success is low. Behavioral and expressive language difficulties are amenable to treatment by androgen therapy and psychological help. Early treatment may be imperative for optimal outcome.

It is the purpose of this presentation to review the unique structure and function of bone marrow anchored hematopoiesis in their significance for its response mechanisms to an exposure to ionizing radiation. The ultimate objective of bone marrow hematopoiesis is to maintain in the peripheral blood a constant level of the different blood cell types (erythrocytes, granulocytes, platelets, lymphocytes, etc.). All of them have their particular turnover kinetics (such as granulocytes 120 × 109/d, erythrocytes 200 × 109/d or thrombocytes 150 × 109/d), are semi-autonomous in their steady state regulatory mechanisms and dependent on a life-long supply of mature cells from a stem cell pool with unlimited replicative and pluripotent differentiative potential. The present knowledge of hematopoietic cellular renewal is the result of years of basic experimental and clinical studies using radionuclides in various metabolic forms including 59Fe, 32P (DF 32P), 51Cr, 131I, 60Co, 3H (3HTdR) and 14C (14CTdR). To understand the physiology but in particular the radiation-pathophysiology, it is essential to recognize in detail the infrastructure of the bone marrow as a distinct unit. Indispensable for a life-long cell production is the capsule of the marrow - the bone cortex -, the arterial supply of blood connected to the sinusoidal microvascular architecture with its sinusoids contorti and recti as well as the central (cell collecting) sinusoids. It is further of importance to recognize the significance of nerval regulation of blood flow, characterized by myelinated and unmyelinated nerve fibers. The type of unique lining cells of the sinusoids is the prerequisite for the cell traffic between the hemopoietic parenchyma and the blood. This in turn cannot be achieved without an alternative opening and closing of the sinusoidal segments which - in turn - requires a rigid long capsule to assure an - in toto - constant volume of each bone marrow unit. If a bone marrow unit is exposed to ionizing radiation, a perturbance of the balance between cellular growth pressure and blood flow dynamics can be observed, resulting in a special type of bone marrow hemorrhage and an "excess cell loss" that may result in an non-thrombopenic exhaustion of the stem cell pool. Of great importance is the question as to the mechanisms that allow the bone marrow hemopoiesis to act as one cell renewal system although the bone marrow units are distributed throughout more than 100 bone marrow areas or units in the skeleton. The observation that "the bone marrow" acts and reacts as "one organ" is due to the regulatory mechanisms: the humeral factors (such as erythropoietins, granulopoietins, thrombopoietins etc.), the nerval factors (central nervous regulation) and cellular factors (continuous migration of stem cells through the blood to assure a sufficient stem cell pool size in each bone marrow "sub-unit"). It should be recalled that the bone marrow functions as a physiological chimera and becomes established by the hematogeneic seeding of stem cells to a mesenchymal matrix during embryogenesis. The repopulation of the bone marrow after partial body irradiation, after strongly inhomogeneous radiation exposure or after total body exposure with stem cell transplantation can well be considered as a repetition of the embryogenesis of bone marrow hemopoiesis with the key element of stem cells migrating via the blood to stromal sites of the marrow prepared to accept stem cells to home and start their replication and differentiation if the micro-environmental quality permits. In summary, the radiation biology of bone marrow hemopoiesis requires a thorough understanding of the physiology and pathophysiology of structure, function and regulation not only of the process of cellular renewal but also of the intricate infrastructure.

Abstract:: We make a new proposal to describe the very low temperature susceptibility of the doped Haldane gap compound Y2BaNi1-xZnxO5. We propose a new mean field model relevant for this compound. The ground state of this mean field model is unconventional because antiferromagnetism coexists with random dimers. We present new susceptibility experiments at very low temperature. We obtain a Curie-Weiss susceptibility χ( T ) ∼ C /(Θ + T ) as expected for antiferromagnetic correlations but we do not obtain a direct signature of antiferromagnetic long range order. We explain how to obtain the “impurity” susceptibility ( T ) by subtracting the Haldane gap contribution to the total susceptibility. In the temperature range [1 K, 300 K] the experimental data are well fitted by T ( T ) = C imp 1 + T imp / T . In the temperature range [100 mK, 1 K] the experimental data are well fitted by T ( T ) = A ln( T / T c ), where T c increases with x. This fit suggests the existence of a finite Néel temperature which is however too small to be probed directly in our experiments. We also obtain a maximum in the temperature dependence of the ac-susceptibility ( T ) which suggests the existence of antiferromagnetic correlations at very low temperature.

We have studied the low temperature relaxation behavior of the octanulcear iron molecular magnet, Fe8. For temperatures below 400mK we observed a crossover to a quantum tunneling regime. In this regime the relaxation is non-exponential and we argue that it results from evolving intermolecular dipole fields. At the very beginning of the relaxation, the magnetization follows a square-root time dependence. A simple model is presented for a longer time range that is in good agreement with the data over 4 decades in time. Numerical calculations of the local field distribution indicate unusual correlation effects.

The concept that androgen alone can provide an effective male contraceptive has been tested in a multicentre, multiphase trial by the World Health Organization. Results from this trial showed that an ester of testosterone, testosterone enanthate (TE), administered at a dose of 200 mg/week, has a very high contraceptive efficacy, and suggested that, at least in some populations, androgen alone might provide a viable option for the control of male fertility. It has been claimed that testosterone represents one of the gender‐related risk factors for coronary artery disease (CAD) in men. Epidemiological and interventional studies have failed to establish a convincing relationship between testosterone and high density lipoprotein cholesterol (HDL‐C). Therefore, there is concern about possible negative effects on lipoprotein asset of an androgen‐alone male contraceptive. In this study we analysed the effects of long‐term (12 months) administration of TE (200 mg/week) in normal healthy men. Blood samples (six men >10 h fast = Group 1; 30 men >4 h fast = Group 2) were drawn from 36 men, monthly before the beginning of the injections (control), every 3 months throughout the study period (treatment), and 1 month after stopping TE injections (recovery). Total cholesterol (chol), triglycerides, HDL‐C and LDL‐C levels were measured in these samples. Biochemical parameters were also monitored. TE administration induced a significant decrease (15–20%) in HDL‐C levels that was of comparable magnitude in men from both groups (fasting and non‐fasting) and occurred regardless of basal HDL‐C levels. No statistically significant effect on other lipoproteins was detected. Considering all men together, HDL‐C levels were decreased in 78% of the men by month 3, 83% by month 6, 94% by month 9 and 97% by month 12 of treatment. In all men the HDL‐C decrease was reversible within 1 month of stopping TE administration. It is concluded that: (1) injection of 200 mg TE/week causes a 15–20% decrease in HDL‐C in normal men with no effect on other lipoproteins, (2) the suppressive effect of TE is maintained throughout the 1‐year‐injection period, and a direct relationship between the duration of TE administration and the proportion of men showing decreased HDL‐C levels, was observed. (3) The HDL‐C decrease was reversible within 1 month of stopping TE administration. These data will be important in designing further studies on male contraception, and in interpreting the relationship between testosterone levels, HDL‐C levels and potential cardiovascular risk.

We present the results of a combined neutron scattering and magnetization study of the pseudo-one-dimensional magnetic compounds [ (CH3)3NH] Co1-xNixCl3?.2 H2O, abbreviated as CoNiTAC. For Co rich samples, we determined the 3d magnetic structure to be canted antiferromagnetic having the magnetic space group Pnm'a'. We performed Monte-Carlo simulations and could reproduce well the measured temperature dependence of the sublattice magnetization assuming an Ising type, spatially strongly anisotropic interaction. The exchange integral along the chains (b-direction) is at least 100 times larger than the average exchange in the a-c-plane. Despite this strong one dimensional character of the exchange interactions the critical exponent ? was found to be very close to the theoretical value for a 3d Ising system (0.31). We did not discover any anomaly of the long range order component for the mixed crystals, for which a spin glass phase had been proposed by other authors on the basis of a decay of the TRM on macroscopic time scales. Instead, our polarized neutron diffraction and magnetization measurements reveal that the low temperature dynamics is connected to domain relaxation processes and not to a spin glass transition.

The high prevalence of men with varicoceles who by history are fertile has led some to question the suggested causal relationship between a varicocele and male infertility. However, testicular function in these fertile men has not previously been studied in detail. Fifty-five normal fertile men, 42 fertile men with varicoceles, and 24 infertile men with varicoceles and normal female partners were studied. Semen analyses were done, baseline serum testosterone and gonadotropin levels tested and the gonadotropin response to luteinizing hormone-releasing hormone (LH-RH) measured. The infertile men with varicoceles exhibited lower sperm counts, abnormal sperm morphologic features, increased baseline serum gonadotropins, and increased gonadotropin responses to LH-RH, compared with the normal fertile men. The fertile men with varicoceles showed similar abnormalities, although this was not statistically significant in all cases. That semen and hormone abnormalities were observed in both the fertile and infertile men with varicoceles suggests that the presence of a varicocele is associated with some degree of primary testicular dysfunction, regardless of present fertility status.

Magnetic relaxation plateaux observed at low temperature in complex systems, are generally attributed to Quantum Tunneling of the Magnetization (QTM). If the experiments are not performed down to low enough temperatures, alternative interpretations can be given in terms of powerlaw distributions resulting from either switching field distributions or/and coupling between switching blocks leading to self-organized criticality. Besides, the first low-temperature/time-dependent magnetization experiments, performed on single sub-micronic particles and on arrays nanoparticles in molecular crystals, are described.

The peritubular tissue of the human testis has been examined by light and electron microscopy in biopsies from 5 normal men, 10 untreated men with hypogonadotrophic hypogonadism and 52 men with oligospermia or azoospermia. In the normal, the lamina propria of the tubules consisted of two alternating layers of collagen fibres and myoid cells to whose surface a thin layer of basement membrane-like material and microfibrils was closely applied. In hypogonadotrophic hypogonadism, the immature seminiferous cords were surrounded by less well defined layers of myoid cells and collagen fibres and the myoid cell surfaces did not exhibit the coating of basement membrane-like material and microfibrils.In the patients with azoospermia or oligospermia, three patterns of organization of the lamina propria were apparent: (1) The normal appearance. (2) Increase in thickness of the layers of collagen fibres. (3) Disorganization of the lamina propria by a marked increase in the thickness of either the basement membrane-like material or microfibrillar layers immediately adjacent to the myoid cells. The third pattern was found in patients with severely damaged testes such as in seminiferous tubule hyalinization. The observations indicate that the thickening of the lamina propria of the seminiferous tubules seen in some damaged testes does not involve the appearance of new components but widening of existing structures.

The luteolytic activity of oxymetholone, and anabolic steroid, has been evaluated in 10 women. Administration early in the follicular phase of the cycle inhibited ovulation and prolonged the duration of the cycles in 2 of 3 subjects, but treatment beginning on Day 10 (3 subjects) did not prevent ovulation, although subsequent plasma progesterone concentrations were reduced. Treatment after ovulation (4 subjects) suppressed progesterone levels by 50 to 80 per cent and shortened cycle length by 6 to 8 days. Side effects were weight gain and bromosulfophthalein retention. The most likely mechanisms producing these perturbations are the inhibition of luteinizing hormone release early in the cycle and, later, inhibition of progesterone biosynthesis.

Magnetization measurements performed below 1 K on a single-crystalline sample of the heavy fermion compound CeRu2Si2 are reported. The field variation of the linear term of the specific heat ? derived from these data (via a Maxwell relation) exhibits a large peak at the metamagnetic-like transition at B* (=7.7 atT ? 0). Thermal-expansion measurements, show a drastic drop of the temperature where a Fermi liquid behavior is reached whenB ?B*. Similar experiments performed on a single-crystalline Ce0.95La0.05Ru2Si2 sample show a broadening of all the anomalies atB* and a reduction of their intensities. These results strongly suggest that pure CeRu2Si2 approaches a magnetic instability atB* forT ? 0.

Abstract:: We observed non-exponential relaxation for a quantum tunneling molecular magnetic system at very low temperatures and argue that it results from evolving intermolecular dipole fields. At the very beginning of the relaxation, the magnetization follows a square-root time dependence. A simple model is developed for the intermediate time range that is in good agreement with the data over 4 decades in time. Detailed numerical calculations as well as measurements are presented which indicate unusual correlation effects in these systems.

In general, fluctuations suppress order in “conventional” many body systems. Here we present results of an experimental study intended to test recent ideas about how quantum, thermal and quenched noise may induce ordering in highly frustrated magnets. Our model system, the garnet Fe2Ca3Ge3O12, is an antiferromagnet with a continuously degenerate Néel ground state. We report combined magnetization- and neutron diffraction studies on the dilution series (FexGa1-x)2Ca3Ge3O12 (0=x=1). We propose a complex magnetic phase diagram with a spin glass phase around x=0.5. Within a wide concentration range, we observe a micro-domain state, where true long range order is never established. The correlation lengths lie between some 300 Å for x=0.4 and =900, Å for x=0.8. We relate our findings to recent theories about how fluctuations select a definite spin structure in highly frustrated antiferromagnets. This leads us to propose two different spin structures for the pure and the diluted samples: our observations are in accordance with a stabilization of a collinear structure via quantum and thermal fluctuations, while quenched stoichiometric disorder lifts the ground state degeneracy but favours a canted spin arrangement.

Fourteen patients with hypogonadotrophic eunuchoidism (HE), 10 males and 4 females, received luteinizing hormone-releasing hormone (LH-RH) as 4-h intravenous infusions of 0.2 μg/min or as subcutaneous (sc) injections of 200 μg. Repeated LH-RH administration over 4 days (200 μg sc daily) was undertaken in 8 patients. Eight of the 10 males and all 4 of the females were found to have definite elevations of luteinizing hormone (LH) and follicle stimulating hormone (FSH) following their first exposure to exogenous LH-RH. The patients included 2 males and 1 female with variant forms of HE, all of whom showed responses of both gonadotrophins.When the first exposure of HE patients to exogenous LH-RH was as a 4-h infusion, the biphasic pattern of LH increase characteristic of normal adults was not found. Instead, a monophasic and qualitatively small LH increase occurred, similar to that found in pre-pubertal children and in anorexia nervosa. In 2 patients with an initially abnormal LH response, a 4-h LH-RH infusion following 4 days of LH-RH injections (200 μg sc/day) produced a normal biphasic LH increase. These results imply that maintenance of the two pools of pituitary LH demonstrable in normal adults depends on adequate production of endogenous LH-RH.Increases in FSH following the initial exposure to LH-RH were generally as great or greater than those of normal adults in spite of the fact that LH responses tended to be smaller than those of adults. Four days of LH-RH administration (200 sc daily) did not lead to consistent increases in gonadotrophin responsiveness. Increases of testosterone or oestradiol production could not be demonstrated, even with 4 to 5 days of LH-RH administration. Exogenous sex hormone therapy markedly inhibited gonadotrophin responsiveness to LH-RH.

Reproductive functions in most animals demonstrate seasonal fluctuations that allow young to be born at a time of the year favourable for their survival. Whether there is a seasonal change in the human reproductive system is unclear. In the present study, we measured serum concentrations of luteinizing hormone, follicle stimulating hormone, testosterone and inhibin in the same 16 normal men sampled monthly for 1 year. A statistically significant increase in all four measured hormones was found in June, with a nadir in August. Our findings suggest that a circannual rhythm of gonadotrophins and testicular hormones exists in normal men. The mechanism leading to this rhythm and the importance of the rhythm in human biology are unknown.

In order to study the ability of the sperm penetration assay (SPA) to correctly classify the fertility status of men, we prospectively examined the results of the SPA performed on the semen of three groups of men of known fertility status. The groups included 67 normal men without varicoceles whose wives were pregnant (VARN), 51 men with a palpable varicocele whose wives were pregnant (VARF), and 30 infertile men with varicoceles (VARI). Two SPAs were done on each subject. Ninety-seven percent of the VARIs showed<15% penetration on a single test, and 91% showed<15% on both tests. On a single test 61% of the VARNs and 68% of the VARFs were<15%. If 0 penetration were used as the criteria of infertility, then 40% of the VARIs, 27% of the VARFs, and 12% of the VARNs would be classified as being infertile. These data suggest that the SPA cannot independently define male fertility status and should be used in conjunction with the standard semen analysis and clinical evaluation of the couple to assess male fertility potential.

Three men who volunteered as normal subjects were found to have abnormally high levels of serum follicle-stimulating hormone (FSH) despite having normal seminal fluid analyses and fertility. Two of the men had a history of previous orchitis, and one had an atrophic testis. Serum luteinizing hormone and testosterone levels were normal. These cases appear to represent compensated primary testicular disease, with normal sperm counts and fertility maintained at the expense of chronically elevated FSH levels. These results imply that in certain situations, the measurement of serum FSH levels may be a more sensitive index of testicular disease than the performance of seminal fluid analyses.

To determine whether there has been a decline in semen quality in a group of healthy men during the past 21 years.

Ocular symptoms developed in four out of 58 patients during clomiphene citrate administration. Ophthalmologic examination revealed scotomata development which disappeared after drug was discontinued in one subject. It recurred with readministration of clomiphene citrate but not with a placebo.The ocular symptoms in three subjects were reversed by discontinuance of clomiphene citrate. The symptoms disappeared while the drug was still being administered to the fourth patient.

Testicular biopsy specimens from fifteen normal adult men were analysed histologically, using Clermont's classification for the germ cell types. After corrections were made for differences in nuclear size and life-span, spermatid/primary spermatocyte ratios ranging from 0·87 to 3·82 (average 2·58) were found. This represents a marked reduction from the theoretical ratio of 4·0 which would be expected. The significance of this attrition rate, or the point at which loss occurs, is not known.

Autoradiographic localization of 125I-labelled luteinizing hormone (LH) in the renal proximal convoluted tubule of rats initiated a study of the role of the kidney in the metabolism of LH. Incubation of 131I-labelled LH with rat renal homogenates for 90 min failed to destroy its immunological reactivity.The plasma half-life of 131I-labelled LH injected i.v. was investigated in normal ewes and rams. Preliminary studies indicated that the use of a preparation containing a high proportion of 'damaged' iodinated hormone could erroneously prolong the plasma half-life. Before use, 131I-labelled LH was purified by gel filtration and column chromatography using DEAE-cellulose. The mean plasma half-life of 131I-labelled LH in normal ewes was 26·7 min and was not significantly altered after oophorectomy (31·1 min). After bilateral oophorectomy and nephrectomy, marked prolongation of the plasma half-life was observed (mean 70·7 min). In normal rams, mean plasma half-life of 131I-labelled LH was 32·0 min, after orchidectomy 44·0 min and after orchidectomy and nephrectomy 82·0 min. The plasma half-life of 131I-labelled LH in a nephrectomized ewe maintained on haemodialysis was also prolonged (67·5 min), the determination being performed 24 h after the last haemodialysis. Although the kidney does not degrade LH, the plasma half-life was significantly increased after nephrectomy. This suggests that the localization of 125I-labelled LH in the proximal convoluted tubule may represent a secretory mechanism allowing renal excretion of LH.

In 1935, Collip and Anderson (1) observed that animals treated with anterior pituitary extracts became refractory after an initial period of stimulation. Serum from refractory animals inhibited the actions of hormone administered to untreated animals. These inhibitory substances have been termed »antihormones«. Most of the information concerning the antihormone phenomenon has been obtained during the past two decades from experiments involving the administration of gonadotrophic hormones. The antihormones which form as a result of administering gonadotrophic hormones are called antigonadotrophins.Antihormones do not form spontaneously; they have not been demonstrated in untreated individuals. Moreover, with rare exceptions, antihormones do not form when recipient and donor of the hormone are of the same species. Antihormones may be passively transferred; the injection of a treated animal's plasma which contains antihormones can prevent the action of the hormone administered to previously untreated animals. The mechanisms governing the formation of antihormones are

The presence of an extra chromosome in the cells of patients with 47,XXY Klinefelter's syndrome and hypogonadism appears to be related phenomena. Testicular histopathology and function were compared in six patients with a 47,XXY karyotype and six patients with a 46,XY/47,XXY karyotype determined upon testicular cells. In addition, the frequency of clinical, histological, and hormonal abnormalities in these two groups of patients was compared following an extensive literature review. A greater degree of damage to the seminiferous tubules and germinal epithelium was found in the 47,XXY patients than in the 46,XY/47,XXY group. These results indicate that the normal 46,XY cells modify the damaging effect to the testis of the 47,XXY cells. This evidence supports a cause-effect relationship between the extra X chromosome and spermatogenesis in Klinefelter's syndrome.

Hypogonadotropic hypogonadism with anosmia has been found in two brothers and a half sister, who are related through the same mother. Although the mother does not have the full syndrome, her history suggests she may have minor symptoms of the syndrome. X-linked inheritance seems most compatible with the familial distributions. Differences of this syndrome from other inherited hypogonadal conditions are discussed. The genetic defect is thought to directly or indirectly affect the hypothalamus which has associations with both olfaction and pituitary function. The importance of testing for anosmia in patients with abnormality in sexual development is noted.

Cylindrical profiles of an AA6063 aluminium alloy were produced in a lab-scale direct extrusion set-up. The extrusion was performed at 300 degC, 450 degC and 550 degC, respectively, with the same ram speed. Immediate water quenching was applied to the profiles and the end of billet (butt-end) after extrusion. Microstructure and texture of the material in different states were measured by electron back-scattered diffraction. Only the profile extruded at 300 degC, was found in the deformed state after extrusion, featuring a fibrous grain structure and a strong <111> and weak <100> double fibre texture. Post-extrusion annealing of this profile at 450 degC resulted in an almost fully recrystallized structure (recrystallized fraction of 87%) and with a texture similar to that of the as-deformed state. The profile extruded at 450 degC was almost fully recrystallized (recrystallization fraction 91%) already after quenching, and with a texture characterized by a weak <111> and strong <100> double fibre. The profile extruded at 550 degC showed a partially recrystallized grain structure with recrystallization fraction of 71%, and with a texture dominated by a <100> fibre. The influence of the deformation conditions on the recrystallization behaviour, in terms of recrystallization kinetics and mechanisms, are discussed in view of these results.

Hysteresis properties of single crystals of a Mn12acetate complex (Mn12Ac) differ from those of other ferromagnetic materials. They are due not to arrangements of magnetic domains, but to a self-induced magnetization reversal, the so-called ‘avalanche’. Using different methods, we present the hysteresis properties of Mn12Ac measured at low temperatures.

Nineteen vasectomized men were followed for 21 to 42 months after surgery, and their sera were tested for the presence of HL-A lymphocytotoxic antibodies. In a previous study, the sera of two of these men had shown a definite increase in serum reactivity 6 to 12 months after surgery. Only one of the nineteen tested in the study demonstrated a single, weakly positive, reaction 24 months after surgery. It was considered that the initial stimulus for lymphocytotoxic antibody production was related to surgery. There was no evidence of antibody stimulation 21 to 44 months postoperatively.