Serum harvested from Sprague-Dawley rats twice vaccinated with gamma-irradiated cercariae of Schistosoma mansoni is able to protect naive recipients against a challenge infection, but the challenge parasites are susceptible to immune elimination over a very short period of time. Thus, vaccinated rat serum protects recipients against a percutaneous cercarial challenge when transferred on Day + 5 but not Day 0 and protects recipients against a tail vein challenge with 5-day-old lung worms when transferred on Days 0 or +1, but not Days +4 or +5. Rats challenged with lung worms via the tail vein and given serum on Day +3 exhibit approximately half the protection expressed by counterparts that received serum on Day 0. However, vaccinated rat serum does not protect naive recipients against a lung worm challenge introduced directly into the liver. These data indicate that immune elimination of challenge parasites in the vaccinated rat model is sitedependent rather than stage-dependent, and most probably occurs during the lung phase of parasite migration.