Normal, and vaccinated mice four weeks previously with radiation attenuated cercariae of S. mansoni (500/mouse) were challenged with normal cercariae (150/mouse), then treated one week later with praziquantel (400 mg/kg body weight, orally). Worm burden was determined to calculate the % immunity in all groups under study. Histopathological examination of liver, small and large intestine, spleen and lung was done. Serum IgE level was estimated using the immunoradiomnetric assay(IRMA). The % immunity was highest among vaccinated, infected and treated group with minimal pathological changes recorded and highest IgE level. From the data collected, it was found that, the efficacy of praziquantel treatment was enhanced in vaccinated mice and that there was synergistic effect between drug treatment and vaccination when paraziquantel was given seven days post-infection (challenge).