and Jonathan Berman From the Consorcio de Investigaciones Bioclinicas, Direccion de

To improve upon the efficacy of primaquine prophylaxis for malaria (94%, Plasmodium falcipa-rum malaria; 85%, Plasmodium vivax malaria), we administered chloroquine (300 mg weekly) in combination with primaquine (30 mg daily) to nonimmune Colombian soldiers during 16 weeks of patrol in a region of endemicity and for a further 1 week in base camp. The occurrence of symptomatic parasitemia was determined during those 17 weeks and during a further 3 weeks in base camp. The protective efficacy for the chloroquine/primaquine treatment group of 100 subjects, compared with that for the placebo treatment group of 51 subjects, was 88 % (95 % confidence interval [CI], 76–94) against all types of malaria, 89 % (95 % CI, 61–97) against P. falciparum malaria, and 88 % (95 % CI, 58–93) against P. vivax malaria. Two chloroquine/primaquine recipients had severe gastrointestinal distress. Comparison of these data with data from a previous study indicates that the addition of chloroquine did not increase the prophylactic efficacy of primaquine. Prophylaxis for malaria is essential for nonimmune individuals who enter regions where malaria is endemic. Although mefloquine and doxycycline are both highly effective as prophylaxis for ma-laria [1], the possibility or actuality of toxicity and drug resistanc