We describe here the participation of a Trichomonas vaginalis 30-kDa proteinase (CP30) with affinity to the HeLa cell surface in attachment of this parasite to host epithelial cells. The CP30 band is a cysteine proteinase because its activity was inhibited by E-64, a thiol proteinase inhibitor. In two-dimensional substrate gel electrophoresis of total extracts of the trichomonad isolate CNCD 147, three spots with proteolytic activity were detected in the 30-kDa region, in the pI range from 4.5 to 5.5. Two of the spots (pI 4.5 and 5.0) bound to the surfaces of fixed HeLa cells corresponding to the CP30 band. The immunoglobulin G fraction of the rabbit anti-CP30 antiserum that recognized a 30-kDa band by Western blotting and immunoprecipitated CP30 specifically inhibited trichomonal cytoadherence to HeLa cell monolayers in a concentration-dependent manner and reacted with CP30 at the parasite surface. CP30 degraded proteins found on the female urogenital tract, including fibronectin, collagen IV, and hemoglobin. Interestingly, CP30 digested fibronectin and collagen IV only at pH levels between 4.5 and 5.0. Moreover, trichomonosis patients whose diagnosis was confirmed by in vitro culture possessed antibody to CP30 in both sera and vaginal washes, and CP30 activity was found in vaginal washes. Our results suggest that surface CP30 is a cysteine proteinase necessary for trichomonal adherence to human epithelial cells.