An endogenous inhibitor of high molecular weight protease was purified from human erythrocytes and partially characterized. The inhibitor was isolated by DEAE-Sephacel ion-exchange chromatography followed by separation on a Bio-Gel A-0.5m column. The inhibitor displayed a native Mr of 240,000 and contained a single subunit of Mr 40,000 after NaDodSO4/polyacrylamide gel electrophoresis. The Mr 240,000 hexamer inhibited high molecular weight protease noncompetitively (Ki = 8.3 X 10(-8) M) and showed marked susceptibility to proteolytic digestion and heat treatment. The purified factor was also a potent inhibitor of calcium-dependent protease (Ki = 2.8 X 10(-8) M), whereas it had no effect on trypsin, chymotrypsin, or papain. Heat treatment (50-70 degrees C X 10 min) caused loss of inhibition against high molecular weight protease; however, inhibition of calcium-dependent protease was stable under the same conditions. This result is consistent with different domains on the inhibitor that interact with high molecular weight protease and calcium-dependent protease. Together with earlier studies in which repression of inhibitor by an ATP-ubiquitin-dependent process was proposed, the present results suggest a general mechanism for regulation of multiple nonlysosomal proteases that are complexed with endogenous inhibitors.