Intraperitoneal administration of leupeptin to rats induced a hemoglobin-hydrolyzing protease which was most active at pH 3.5 and was insensitive to pepstatin in various tissues such as the liver, kidney, and muscle, as observed previously in adult rat hepatocytes in primary culture (Tanaka, K., Ikegaki, N., and Ichihara, A. (1979) Biochem. Biophys. Res. Commun. 91, 102-107). The induced acidic protease was purified about 600-fold in 30% yield from rat liver by conventional chromatographic techniques. The purified enzyme appeared homogeneous by polyacrylamide gel electrophoresis in the presence or absence of sodium dodecyl sulfate and was a monomeric protein of Mr = 20,000. The enzyme appeared to be a glycoprotein because its induction was blocked by the addition of tunicamycin to cultures of hepatocytes and because the induced protease was absorbed on concanavalin A-Sepharose and eluted with methylglucoside. It seemed to be present in lysosomes and was fairly stable at various pH values and temperatures. It showed endopeptidase activity on various protein substrates, but scarcely hydrolyzed N-substituted derivatives of arginine. It did not hydrolyze esters, showed no aminopeptidase or carboxypeptidase activity, and did not inactivate glucose-6-phosphate dehydrogenase or aldolase. The enzyme appeared to be a thiol protease, since it was strongly inhibited by sulfhydryl-reactive compounds and N-( [N-(1-3-trans-carboxyoxiran-2-carbonyl)-L-leucyl]-agmatine and was not inhibited by reagents specific for carboxyl-, serine-, or metalloproteases. This induced protease could be separated from cathepsins B, D, and H by chromatography. The enzyme was similar to cathepsin L in chromatographic behavior, Mr and pI, but differed from the latter in stability and in its inability to inactivate some enzymes. These results suggest that it differs from any known proteases found previously in rat liver.