A partially active and a latent form of multicatalytic protease (MCP) were isolated from fish skeletal muscle. Both forms were inactive against protein substrates, but their activity against peptide substrates differed in one order of magnitude. The chymotrypsin-like activity of the partially active form was moderately stimulated by fatty acids and SDS, whereas its trypsin-like activity was inhibited by the same reagents. In contrast, both activities of the latent form were strongly stimulated by SDS. The chymotrypsin-like activity of the latent form was also stimulated by heating or high urea concentrations, whereas its trypsin-like activity did not change or was inhibited respectively by these treatments. These activation effects were irreversible. Pre-treatment of the latent form with SDS or urea in the absence of substrate led to its irreversible inactivation, whereas activation by pre-heating occurred in the presence or absence of substrate. These results suggest that MCP can exist in several active states with distinct properties. Studies on the distribution of MCP in fish tissues showed a much higher level of the enzyme in gonads than in any other tissue, suggesting a role of MCP in development.