Our previous studies have shown that riboflavin has activity against Plasmodium falciparum asexual-stage parasites in vitro, In the present study we examine the gametocytocidal activity of riboflavin and the interaction of riboflavin with some commonly used antimalarial drugs against the asexual forms of P, falciparum in vitro. The addition of riboflavin to P, falciparum cultures killed gametocytes at all stages, even those at late stages (III to V), which are not affected by many of the commonly used antimalarials. Combinations of riboflavin with mefloquine, pyrimethamine, and quinine showed a marked potentiation of the activities of these drugs against asexual-stage parasites in vitro. The combination of riboflavin with artemisinin was additive, while that with chloroquine was mildly antagonistic. High doses of riboflavin are used clinically to treat several inborn errors of metabolism with no adverse side effects. Its efficacy in combination with standard antimalarial drugs in treating and preventing the transmission of P. falciparum malaria can therefore be evaluated in humans.