This report summarizes the results of appraisals of various activities of CI-679 (a 2,4-diamino-6-amino-substituted quinazoline) in rhesus monkeys infected with the Ro and Ro/PM strains of Plasmodium cynomolgi. In subjects inoculated with sporozoites, CI-679, administered in appropriate schedules in doses up to and including the maximum tolerated level, neither prevented development of infections with these strains nor cured those already established. Although these evaluations showed that CI-679 was devoid of activity against both early and late tissue schizonts, they indicated that this compound had significant blood schizonticidal activity and suggested that such activity was much greater in infections with the Ro strain than in those with the Ro/PM strain. This latter suggestion, contrary to results of earlier experiments in mice infected with Plasmodium berghei, led to studies of the activities of CI-679 in trophozoite-induced infections. These showed that the total course dose of CI-679 required for cure of previously untreated infections with the Ro/PM strain was tenfold that required for cure of comparable infections with the Ro strain. They also showed that infections with either strain, treated previously with subcurative doses, were often not cured by doses of CI-679 that eradicated previously untreated infections regularly. Subsequent studies showed that emergency of parasites resistant to CI-679 was responsible for these retreatment failures and that this resistance was retained through mosquito transfer.