The migration of Schistosoma mansoni (LE and SJ strains) has been studied in eight groups of outbred Swiss albino mice (Mus musculus), which were previously infected with ca 450 cercariae, transcutaneously. The infection of mice was performed with non irradiated cercariae (control groups), or with gamma-irradiated cercariae, at the schedule of 3, 20 and 40 Krad. Regarding the skin, a progressive decrease was detected for the recovery rates, related to the time of infection. As far as the lungs and portal system are concerned, a significant inverse correlation was observed between the total recovery rate and the irradiation dosages. The dose of 20 Krad practically hinders the migration of the parasites (in both strains) from the lungs to the portal system, whereas the dose of 40 Krad prevents the migration of most of the parasites from the skin to the lungs.