Seven proteases assumed to be aminopeptidases A, B and M, dipeptidyl peptidases II and IV, esteroproteinase and gamma-glutamyltransferase were localized histochemically, using semipermeable membrane simultaneous coupling techniques, in unfixed cryostat sections of skeletal muscle removed from one healthy volunteer, six patients with disuse muscle atrophy, and 15 patients with some form of muscle disease. Normal muscle fibres showed weak reactions for aminopeptidases A and M and for the dipeptidyl peptidases, but no reactivity for gamma-glutamyltransferase or esteroproteinase. No change was detected in diseased muscle fibres except that low gamma-glutamyltransferase and esteroproteinase activities appeared in some cases. The activities of the seven enzymes were stronger in the intermyosial connective tissue than in the muscle fibres, but were also unchanged in disease. The strongest reactions were found in some interstitial cells (mast cells and macrophages) and these were much increased in diseased muscle, particularly for dipeptidyl peptidases II and IV. The findings are interpreted in terms of the release of proteases from such cells and their subsequent involvement in the breakdown of myofibrillar proteins in muscle disease.