Experiments were performed to determine the level of protection in immunized mice to multiple small (trickle) Schistosoma mansoni cercarial challenges. C57BL/6 mice were immunized with either 50 Krad-irradiated cercariae or a soluble worm antigenic extract injected intradermally in conjunction with the adjuvant BCG. A series of cercarial challenges (40 cercariae per exposure), beginning approximately one month after immunization, was given at 4-week intervals. To prevent any contribution of egg-related pathology, the mice were exposed to male cercariae only. The results showed that highly significant levels of protection developed at all times after cercarial challenge. Regardless of the type of immunization or the number of cercarial challenges, the levels of protection which appeared after one, two, three, or four challenge exposures were nearly identical, and equalled those in mice exposed at a single time to a cumulative total of the several exposures. As expected, titers of anti-schistosome antibodies were higher in the immunized and challenged groups than in those challenged alone, and titers increased anamnestically after challenge infection. However, increased antibody titers were not associated with increased resistance in trickle challenged mice. Overall, we showed that the level of vaccine-induced immunity does not change, under these conditions, in the face of repeated cercarial challenges. Also, these results show that single mass cercarial challenges may legitimately be used in experimental situations to assess the effectiveness of vaccines which might be used in subjects repeatedly exposed to cercariae in field situations.