Human alpha-1-antitrypsin (alpha 1-AT) was incubated with an extract of Schistosoma mansoni cercariae or porcine pancreatic elastase and analysed by immunoelectrophoresis and Western blotting. The inhibitor was shown to form complexes with components in S. mansoni cercariae in the same way as elastase. The role of alpha 1-AT in S. mansoni infection is discussed.