At particular doses of gamma radiation from Co60 source suppresses the intracellular parasitism, a fact which raises the question of whether the phenomenon may find practical applications in the outcome of infectious diseases. In this study, stationary phase of promastigotes exposed to radiation doses in the range 10-20 krad (standard dose) resulted in significant parasitization of mononuclear phagocytic system in vitro. However, promastigotes irradiated with 20krad consistently resulted in higher parasitization and optimum infection after acquiring the shape of amastigote- like organism than those exposed to either higher or lower radiation doses. It was observed that 10 krad was necessary to immobilize immediately the organisms, whereas only 30 krad rendered them noninfective and 40-50 krad made promastigotes unable to recognize the binding ligand for attachment to macrophage cell. In comparison to the irradiated parasite, the rate of phagocytosis of 20 krad irradiated cells, were higher while considering the percentage of infected macrophages, the mean number of engulfed parasites by each macrophage cells and the statistics of the two. The rate of infected cells was approximately 8% greater in 20 krad irradiated cells than nonirradiated cells.