Human neurocysticercosis, due to infestation of the central nervous system with Taenia cysts, is a common cause of neurologic disease in endemic areas and is being increasingly recognized in the United States. Previous studies have suggested that Taenia cysts bind host IgG via Fc-like receptors and that bound IgG is degraded by the parasite, perhaps as a source of nutrients or a means of immune evasion. We now demonstrate that IgG degradation is thiol dependent and is inhibited by the cysteine proteinase inhibitor, E-64. The cysteine proteinase activity from Taenia crassiceps cysts was purified 682-fold by acid extraction, gel filtration chromatography, and anion-exchange FPLC. The cysteine proteinase appeared as a 43 kDa band on silver-stained gels. Its isoelectric point was 5.27. The purified enzyme was inhibited by cysteine proteinase inhibitors and also by chloromethyl ketone inhibitors, but not significantly by other inhibitors of serine, aspartic or metallo-proteinases. Substrate studies showed pronounced cleavage of Z-Phe-Arg-7-amino-4-trifluoromethylcoumarin (Z-Phe-Arg-AFC), but not of substrates with neutral or positively charged amino acids in the P2 position. K(m) for Z-Phe-Arg-AFC was 1.0 x 10-7 M, K(cat) 58 s-1 , and K(cat)/K(m) 5.8 x 108 mol-1 s-1. Amino acid sequencing of the amino terminus revealed a single cysteine residue with surrounding residues suggestive of a cysteine proteinase active site. The sequence, however, did not contain the conserved active site associated with enzymes of known cysteine proteinase families. This cysteine proteinase may play an important role in the interaction of Taenia cysts and host immunoglobulin and is a potential target for antiparasitic chemotherapy.