Studies with 3 volunteers were conducted to determine the effects of a combination of sulfadiazine and pyrimethamine and the effects of primaquine upon mature gametocytes of a strain of chloroquine-resistant Plasmodium falciparum-the Malayan (Camp.) strain. One volunteer was treated with sulfadiazine and pyrimethamine; two other volunteers each received a single dose of 45 mg of primaquine base. The combination of sulfadiazine and pyrimethamine, although active against blood schizonts, did not exert a marked sporontocidal effect against the Malayan (Camp.) strain. In sharp contrast, primaquine, although not effective as a blood schizontocide, exerted a marked gametocytocidal and sporontocidal effect against this strain.The findings emphasize the need for further studies of the sporontocidal and gametocytocidal effects of drugs, particularly primaquine, against chloroquine-resistant strains of P. falciparum and suggest that primaquine may come to play an important role in preventing the transmission of such strains.