Potential mechanisms causing elimination of lung stage schistosomula were investigated in normal C57BL/6 mice and C57BL/6 mice previously vaccinated with irradiated cercariae. Autoradiography was used to measure the proportion of radiolabeled schistosomula which could be recovered from lung tissue by mincing and incubation. There was no difference between the two sets of mice at each time point, and no substantial diminution in the proportion recovered over the sampling period of 7 to 17 days postinfection. By this in vitro criterion, the emergence of schistosomula from lung fragments was not influenced by inflammation. However, we are unable to rule out the possibility that migration is impeded by inflammation in vivo. The migratory potential and viability of schistosomula recovered after residence in the lungs for increasing lengths of time was assessed by their introduction into the vasculature of naive recipients. The results indicate that significant numbers of schistosomula are capable of maturation, but fail to mature if left in situ in the lungs of both normal and vaccinated mice. No evidence was found that schistosomula suffer cytotoxic injury in the skin, subsequently dying in the lungs, or that they are damaged by the pulmonary inflammatory reactions which develop in vaccinated mice. Schistosomula delivered to the alveoli have a limited capacity to reenter tissues and mature. We suggest that in normal C57BL/6 mice the deflection of parasites into the alveoli during migration is the reason why many fail to mature. In vaccinated C57BL/6 mice a similar but augmented process may account for the elimination of a greater proportion of challenge parasites.