Serum from mice vaccinated with Irradiated cercariae of Schistosoma mansoni (20Krad) was administered in different doses and at different time intervals to naive mice. It was found that the level of protection achieved (% immunity) depended on the volume of serum administered and Its administration pre & post-challenge with normal cercariae the highest percentage immunity recorded (67-68%) was In group five which had received immune serum 1 h before & 1 and 48 hours after challenge (1.5 ml). The lowest percentage Immunity was in group IV (17-19%) which had received immune serum 5 & 6 days post- challenge ( 1 ml) and that Indicates that passive Immunization acts mainly upon skin stages of the parasite and not lung stages. J.Egypt Med.Micro., 3(2):371-374