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SUMMARY Normal human leucocytes, successively treated with glutaraldehyde-tannic acid-osmium tetroxide-thiocarbohydrazide-osmium tetroxide-thiocarbohydrazide-osmium tetroxide, were prepared for scanning electron microscopy observation. These cells produced well-contrasted, non-charging scanning images compatible with metal-evaporated material. Further, the mononuclear and polymorphonuclear cells resisted shrinkage during dehydration and critical point drying, thus allowing much improved images at high magnification than those covered with evaporated metal. In all cases at least a second thiocarbohydrazide-osmium tetroxide treatment could not be avoided.

SUMMARY Leucocytes were isolated from the peripheral blood of normal individuals and prepared for SEM observation. The addition of the buffer Hanks and Hepes to the glutaraldehyde fixative was found to maintain the initial ambient pH throughout cell manipulation, and to allow satisfactory preservation of surface architecture.

The fine structure os Strongylocentrotus purpuratis testes has been examined. No obviously important differences appear to exist between the description reported here and the published fine structure of testes obtained from other sources. The most useful fixative was found to be a mixture of glutaraldehyde-paraformaldehyde.

Variations in the mode of preparation, type of fixation and leukocyte collection can plan an important part in the determination of the surface structure of a given population, and can result in modifications of the microprojections, and their number, evident on the cell surface. We present preparatory modes that circumvent these difficulties, and the results obtained clearly show that the techniques are profitable, appear essential for the preservation of cell population and integrity, and allow accurate scanning electron microscopy observations without resort to time consuming procedures. Indeed, the entire process can be completed in one day.

The fine structure of Tripneustes esculentus eggs was studied with the aid of an electron microscope. Cells obtained from this West Indies sea urchin showed cortical granules, mitochondria forming a rosette around lipid granules, endoplasmic reticula, Golgi apparatus and annulated lamellae. These structures appear identical to those seen in eggs of the Atlantic sea urchin: Arbacia punctulata.