Suspensions of leishmanias from the spleen of hamsters infected with Leishmania donovani were placed in culture flasks and incubated at 37°C. In a medium of human erythrocyte extract and human serum there appeared within a day or two aflagellate forms resembling leishmanias but larger, as well as other aflagellate forms more nearly resembling rounded leptomonads. These intermediate forms multiplied during the first 4 days of culture. They then slowly died off, despite frequent renewal of the culture medium. Sometimes a small proportion of motile, typical leptomonads also appeared in such cultures. Leptomonads from cultures maintained at 28°C., when placed in the human red cell extract-human serum medium and incubated at 37°C., survived at least 4 days. For both types of effect, human serum could be replaced by normal hamster serum but not by rabbit serum.

Nicotinamide, added to the human red cell extract-human serum medium at a concentration of 400 mg. per 100 ml., completely prevented the development of intermediate forms from leishmanias and brought about the rapid death of leptomonads at 37°C.

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