Three 7-chlorinated lincomycin derivatives exhibited curative activity against blood-inoculated Plasmodium cynomolgi infection in rhesus monkeys. Acute infections were treated by oral or subcutaneous administration for 5 consecutive days with doses ranging from 6.25 to 100 mg per kg body weight. Cures were obtained when U-21,251F (7-chlorolincomycin) and U-24,729A (7-chloro N-demethyl-4′-pentyllincomycin) were given orally at 50 mg per kg and when U-26,285A (7-chloro N-demethyllincomycin) was given at 100 mg per kg. U-24,729A and U-26,285A were also curative when given subcutaneously at 25 and 50 mg per kg, respectively. With all dosage regimens the blood was cleared of parasites 3 to 6 days after cessation of treatment. The rate of clearance of parasites did not appear to be related to dose. No side-effects due to the antibiotics were observed.