Abstract: Ketotifen 0.7～20 mg/kg×3 days were given orally to P. cynomolgi infected Maecaca mulatta. The asexual parasites and gametocytes in blood were eliminated within 93～102.5 hours and 102.5～120 hours after administration, respectively. When chloroquine 12 mg/kg×3 days was given, however, parasites disappeared in 66 hours. No effect on the tissuestage of this parasite was observed. If ketotifen 0.1mg/kg×3 days was given in combination with sulfadoxin (SDM') 5 mg/kg×3 days, the asexual parasites and gametoeytes disappeared in 79.5 and 153 hours respectively after administration, indicating a synergistic action between these two drugs. No side-effect was observed during the treatments.In 30 days following observation one of the two monkeys treated with 5 mg/kg ×3 days and two monkeys treated with 1 mg/kg×3 days ketotifen recrudescences of blood parasites emerged.The results showed that ketotifen is a new chemical structure type and effective anti-simian-malaria drug, and the observation of its antimalarial effect against human malaria is undertaking.