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The sensitivity of Plasmodium falciparum strains to chloroquine was tested in one locality in the north-east of the United Republic of Tanzania, where a chloroquine-medicated salt project has been implemented for chemosuppression for many years, and where large amounts of the drug have been available during the last decade for the treatment of malaria infections.Single doses of chloroquine (5 or 10 mg of base/kg of body weight) failed to clear P. falciparum trophozoites in asymptomatic parasite carriers selected from the school population. In comparison, clearance had been obtained easily ten years previously with 5 mg of base/kg of body weight in several localities in the area.A total dose of 25 mg of base/kg of body weight given over a 3-day period succeeded in clearing asexual parasites from the peripheral blood by day 3 in all instances. Asexual parasites were not found again during the nine days following administration of the drug.All the schoolchildren who had received 5 or 10 mg of base/kg of body weight at the beginning of the trial were treated with a further 20 mg of base/kg of body weight at the end of the 7-day observation period. Asexual parasites reappeared in the blood of some of these children 7-10 days after the second administration of the drug.Using the in vitro microtechnique, incomplete schizont inhibition was observed in 3 out of 21 cases at a chloroquine concentration of 1.14 mumol/litre of blood, which is the discriminating dosage for resistance at RI level.No cases of retinopathy related to the prolonged use of chloroquine were detected among the 221 residents who had spent more than 16 consecutive years in the locality.