Chloroquine-resistant infections of malaria are prevalent in many areas of Southeast Asia and South America and their importation into the United States will remain a problem during the foreseeable future. The sensitivity of Plasmodium falciparum to chloroquine and other antimalarial drugs can be determined by a simple in vitro technique. It provides an expedient method for identifying drug-resistant strains and is useful in assessing the blood schizontocidal activity of some antimalarial agents. During studies with the recently solated Vietnam (Marks) strain of P falciparum, parasites showed a marked degree of resistance to chloroquine in vivo and in vitro, but they were significantly more susceptible to amodiaquine. The findings suggest that some P falciparum infections resistant to chloroquine may respond more favorably to treatment with another 4-aminoquinoline.