Experiments were conducted to determine the effect of anti-schistosomal vaccine exposure on the level of pre-existing resistance in mice with a bisexual Schistosoma mansoni infection. C57BL/6 mice with light S. mansoni infections of 8 weeks duration were injected with 10 Krad-irradiated, cryopreserved and thawed schistosomules. Four weeks later the mice were exposed to normal cercariae, and adult worms collected by perfusion 6 weeks post-challenge. In 4 experiments, the baseline levels of resistance in infected mice ranged from 19% to 50% reduction in challenge worm burden (mean of 36%). Although vaccine administration slightly raised the overall level of resistance in infected mice (mean of 49%), in only 1 experiment was the increase over that in infected mice statistically significant. The levels of resistance attributed to the patent infection and vaccine exposure were not additive. Vaccine exposure had no effect on recovery of the adult worms of the primary infection.