Proteolytic activities in crude extracts and culture filtrates from Trichomonas tenax were determined using hide powder azure as substrate and the proteinase profiles in both samples were analysed in SDS-polyacrylamide gels containing copolymerized gelatin. The enzyme activity in the crude extract was detected over a broad pH range and was strongly activated by dithiothreitol, mainly in the pH range 5–8, and inhibited by cysteine proteinase inhibitors. Extracellular enzyme activity in culture filtrates was SH-dependent and increased continuously during incubation of the cell suspension, suggesting proteinase release. A total of seven distinct proteolytic bands could be detected in crude preparations. Three of these, with apparent M r values 35,000, 45,000 and 56,000 and a pH optimum of 4–7, were SH-dependent and their inhibitory sensitivities were characteristic for cysteine proteinases. The 45,000 and 56,000 proteinases probably corresponded to those found in the culture filtrates. Proteolytic bands with apparent M r 76,000, 87,000, 102,000 and 270,000 and pH optima in the alkaline region, pH 8–9, were independent of SH groups and were inhibited by a chelating agent EDTA, suggesting that they belong to the metalloproteinase family.