Separate thermostable inhibitors of cathepsins B and H (IB and 1 , ) were found in every rat and human tissue that was analyzed. The inhibitors were separated from the cathepsins by heating at 80-90 "C at pH 3. I B and I H from rat lung were purified and found to occur in multiple molecular forms. Rat lung IB and IH were very similar proteins with molecular weights of 14000. 1s and I H from hog kidney were also purified and IH was obtained free of IB. These inhibitors also displayed microheterogeneity and had molecular weights of 11000. Hog kidney IH inhibited chymopapain, but did not inhibit bromelain, chymotrypsin or cathepsin D. Rat and human serum contained thermostable inhibitors of cathepsins B and H, but these molecules had relatively high molecular weights. Therefore the low-molecular-weight 1s and IH from other tissues appear to be intracellular in origin. When extracts of kidney or liver were autolyzed at pH 3 -6, there was a large increase in cathepsin B or H activity with a concomitant decrease in I B and IH. Inhibitor destruction was apparently catalyzed by a proteinase other than cathepsin D or E. Rat kidney, liver, and spleen were rich sources of cathepsins B and H, the kidney yielding three times as much activity per gram as liver or spleen. Hog kidney was rich in cathepsin H, but yielded little or no cathepsin B. IB and 1H were also present in protozoa, tuna fish, chicken and toad. The inhibitors from Tetvahi,menapyriformis inhibited the thiol proteinase from this protozoan.