The metacestodes of Taenia pisiformis have been shown to contain a protease inhibitor capable of inactivating the esterolysis of N-α-benzoyl-L-arginine ethyl ester (BAEE) and N-benzoyl-L-tyrosine ethyl ester (BTEE) by trypsin and chymotrypsin, respectively, of bovine, dog and rabbit origin, but not affecting the hydrolytic activity of subtilisin, elastase, collagenase, pepsin, rennin and papain. This inhibitor has been demonstrated in whole worm extracts and in the incubation medium of in vitro-maintained, intact living metacestodes. The protease inhibitor which was purified by trichloroacetic acid precipitation, Sephadex G–100 chromatography and affinity chromatography on CNBr-activated Sepharose 4B–bovine chymotrypsin conjugate was soluble in 5% trichloroacetic acid, withstood heat up to 80°C, tolerated the pH range 1·5 to 9·0, was unaffected by 8 M urea or 0·2 m 2-mercaptoethanol and had a molecular weight of about 7000 to 7200, as calculated from its gel chromatographic behaviour. Complex formation between the inhibitor and the enzymes required 3–4 min for completion. The enzyme–inhibitor complex was not dissociated by 4 m KC1. Activity determinations on bovine TPCK–trypsin and bovine chymotrypsin with BAEE and BTEE assays revealed that the inhibitory actions toward both enzymes are functions of the same or closely adjacent sites of the inhibitor molecule. The supposed function of the inhibitor is discussed.