Abstract: When cycloleucine(80mg/kg) was given to mice infected with Plasmoaium cergnei for five days, it completely suppressed the growth of parasites. The growth of P. falci-parum in vitro in cycloleucine (1.5×10-2M) treated group was considerably arrested at the trophozoite stage, after 72 hours incubation. Abnormal parasites constituted about 90% of the total parasite population.After incubating for 30 hours 1.5×10-2M cycloleucine inhibited the total incorporation of 3H-hypoxanthine by 52-58%, whereas it exhibited a stronger inhibition by 79% in the incorpoation of this compound into nucleic acids.Cycloleucine also inhibited the total incorporation of 14C-methionine by 30% and the synthesis of spermidine by 50%, of which the aminopropyl group was derived from 14C-methionine.The observed effects of cycloleucine on malaria parasites led us to suggest that cyloleucine not only blocked the incorporation of hypoxanthine and methionine but also influenced the metabolism of S-adenosyl-methionine.