Various lines of evidence suggest that calcium dependent protease (CANP, calpain) exists in the cytosol as an inactive proenzyme which is converted to an active form by autolysis. During autolysis only the N-terminal regions of both subunits of proCANP are modified. About 20 and 90 residues are removed from the large and small subunits, respectively. The N-terminal region (domain I) of the large subunit modified during autolysis precedes the protease domain and corresponds to the propeptides of various cysteine proteinases. Analyses of the autocatalytic activation of CANP in the presence of plasma membranes reveal that proCANP translocates to the membrane in the presence of microM Ca2+ and is activated at the membrane. The CANP inhibitor and Ca2+ are the most important factors for the regulation of CANP activity. The primary translation product of the mRNA for rabbit liver CANP inhibitor contains four internal repeats. Structural analyses of the liver and erythrocyte inhibitors reveal that they contain four and three repeats, respectively. The repeating unit was identified as the functional unit of the inhibitor and each unit inhibits one mole of CANP. On the basis of these results, an activation mechanism for proCANP at the membrane was proposed. The native enzyme, which has been called CANP or calpain, should now be called proCANP or calpainogen. CANP and calpain should be used for the autolyzed active form.