The penetration of cercariae through the skin initiates infection of the host with the human trematode parasite Schistosoma mansoni. Many larvae fail to migrate into the living epidermal cell layer. In order to determine if chemical as well as mechanical barriers to cercarial skin penetration exist, inhibitory activity of epidermal cell extracts against the proteinase obtained from cercarial secretions was assayed. An inhibitor was purified 50-fold by gel filtration on Sephadex G 75 and cation exchange chromatography at pH 5.8 and 4.9. The inhibitor has a relative molecular mass (Mr) of approx. 40 000-53 000. Oxidation of the inhibitor with N-chlorosuccinimide eliminated its inhibitory activity and thus indicated a critical methionine residue. The inhibitor was active against a wide spectrum of serine proteinases: porcine pancreatic elastase, human granulocyte elastase, bovine trypsin, and bovine alpha-chymotrypsin. However, no inhibition was detected against papain or clostridial collagenase. The inhibitor did not cross react with antiserum to human or rat serum alpha 1-proteinase inhibitor.