Chemicals with antimalarial effects show different patterns against ring stages, trophozoites, and schizonts of Plasmodium falciparum in culture. Amodiaquine, mefloquine, quinine, quinacrine, primaquine, qinghaosu, halofantrine, rifampin, antimycin A1, cycloheximide, and actinomycin D were preferentially toxic to the mature stages. Ammonium chloride killed ring stages faster than mature forms, while tetracycline, clindamycin, and halofuginone acted with equal speed on all 3 stages.