Since 1948, primaquine, a close relative of pamaquine, the historically important curative antimalarial agent, has been the subject of intensive clinical study at Stateville Penitentiary (Edgcomb et al., 1950; Hockwald et al., 1952; Clayman et al., 1952). More recently primaquine has been used on a large scale among military personnel (Alving et al., 1952). These studies have shown that primaquine is safe and practical for mass prophylaxis and therapy of malaria in nonimmune Caucasians and Negroes in a nonendemic area, and as such is the drug of choice for curative treatment of relapsing malaria (Alving et al., 1953). There remain, however, two very important problems: (1) testing the tolerance of tropical native peoples to primaquine and (2) measuring the response of field infections to treatment with that drug. These problems involve several factors not present in previous studies. Very few tropical populations are well nourished.