Although the fine structure of malarial parasites has been studied by many investigators, there remain many unanswered questions. Re-examination of the morphology of the parasites by the high voltage and scanning electron microscopes has been rewarding for understanding of the various subcellular organelles of malarial parasites. During the past year, we studied by electron microscopy the effect of immune serum on sporozoites of P. cynomolgi and P. berghei. When the sporozoites were incubated in immune serum, the surface coat became very thickened, while normal serum showed little effect on the sporozoite. Scanning electron microscopy indicated that the anterior end of the parasites was free of coat deposition and that the circum-sporozoite precipitate (CSP) reaction was resulted from coat deposition at the posterior end of the sporozoites. For the coming year, we will study microgametogenesis of P. gallinaceum and P. berghei and the effect of immune serum on the microgametes by electron microscopy. BIBLIOGRAPHIC REFERENCES: Aikawa, M., Komato, Y. and Asai, T., Transmission and scanning electron microscopy of host cell entry by Toxoplasma gondii. Am. J. Path. (In Press) 1977. Aikawa, M., Variations in structure and function during the life cycle of malarial parasites. (review) WHO (In Press) 1977.