The erythrocytic stages of Plasmodium lophurae were freed from their host red cells by specific hemolysis directly into a favorable medium containing an extract of duck erythrocytes. Extracellular survival and development of the parasite in vitro occurred in culture media consisting essentially of a very concentrated extract of duck red cells prepared in a special nutrient solution. Omission or dilution of the red cell extract resulted in rapid degeneration of the parasites. Their survival and development were favored by the presence in the erythrocyte extract of gelatin, yeast adenylic acid, and cozymase, and especially by the further addition of adenosinetriphosphate and sodium pyruvate. Under the best conditions yet tested, all the free parasites continued their development extracellularly during the first two days of cultivation. Merozoites formed by the extracellular segmentation of the free parasites originally present developed further into trophozoites. On the third day a majority of the free parasites were still of normal appearance, but by the fourth day more were degenerate, and very few normal parasites remained on the fifth day.

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