Naive CBA/Ca mice and mice vaccinated with gamma-irradiated cercariae of Schistosoma mansoni were challenged percutaneously with normal cercariae and depleted of L3T4+ T helper cells through the administration of a specific monoclonal antibody. Three regimes were utilized to target known phases of parasite migration. The in vivo depletion of L3T4+ cells resulted in a significant reduction in immunity (up to 65%) in vaccinated/challenged mice, provided the monoclonal antibody was targeted towards skin-resident schistosomula. When antibody was targeted towards lung phase challenge larvae, however, there was a significant reduction in worm recovery, but no correspondingly significant reduction in vaccine immunity. In contrast, the administration of monoclonal to naive mice, via all three treatment regimes, had no effect on the primary schistosome worm burden. Histopathological studies complemented these worm recovery data. Skin tissue biopsied from vaccinated/challenged mice treated with monoclonal to L3T4+ T cells rarely showed the inflammatory foci which normally characterize untreated vaccinated/challenged mice. This was true when antibody was given either before challenge, or just after challenge, and correlated with the recorded depression in vaccine immunity. Lung tissue collected from monoclonal-treated vaccinated/challenged mice (for all three treatment regimes) exhibited no changes in morphology compared to that from untreated vaccinated/challenged mice. This was not altogether surprising since in the NIMR vaccine mouse model, the lungs represent a poor site for challenge attrition and appear normal in morphology with the exception of a few, small inflammatory reactions. When the monoclonal was given to naive/infected mice, there was no change in the morphology of the pulmonary tissue, as compared to corresponding untreated cohorts. Immunohistochemical studies revealed that Thy-1+ cells dominated the subdermal inflammatory foci of vaccinated/challenged mice. Of the T cells identified, the T helper subset was the most common, with T suppressor cells being only weakly represented, and in some cases not at all. The proportion of macrophages (Mac-1+) varied between reactions.