Summary A study of the therapeutic effectiveness and toxicity of primaquine in daily doses of 10, 15, 20, and 30 mg. (base) daily for 7 or 14 days, administered concurrently with 1.5 gm. (base) of chloroquine in 3 days, at the time of an acute attack of vivax malaria of Korean origin is reported. The relapse rate in patients treated with chloroquine alone was 43.8 per cent. The relapse rate was reduced to less than 3 per cent by all regimens of primaquine used. These results indicate that a daily dose of 10 mg. of primaquine administered for 14 consecutive days is adequate for the curative treatment of vivax malaria of Korean origin. A daily dose of 20 mg. administered for 7 days was effective in reducing the relapse rate. No significant toxicity was observed with doses of 10 and 15 mg. daily. A daily dose of 20 mg. of primaquine produced acute severe intravascular hemolysis in one of 14 Negroes. This dose was well tolerated by all other patients. Mild abdominal pain and cyanosis due to methemoglobinemia occurred in approximately one-fourth of the white patients treated with 30 mg. of primaquine daily, but these effects were not severe enough to interfere with normal activity.