Chondroitin, chondroitin 6-sulphate, chondroitin 4-sulphate and dermatan sulphate proteoglycans were immunolocalized by monoclonal antibodies applied to human muscle sections digested with chondroitinase. In normal muscle the 4 proteoglycans presented a different extracellular localization: unsulphated chondroitin sulphate (chondroitin) was present in the endomysium and around capillaries, chondroitin 6-sulphate in the basal membrane zone, chondroitin 4-sulphate in the vessel adventitia, in the endomysium around capillaries and, to a lesser degree, in the perimysium, dermatan sulphate in the perimysium and, to a lesser extent, in the vessel adventitia and in the endomysium around capillaries. The enlarged endomysium of pathological muscle contained chondroitin and chondroitin 4-sulphate. Chondroitin 6-sulphate and dermatan sulphate did not seem present in the increased connective tissue. No peculiar pattern was observed in the various neuromuscular diseases studied. The specific extracellular distribution, the different biochemical composition and the different ability to bind to other extracellular components suggest a different biological role of these compounds. Chondroitin 6-sulphate is a component of a highly specialized extracellular structure, namely basal membrane. Chondroitin and chondroitin 4-sulphate participate in the composition of actively changing extracellular matrix such as the endomysium in pathological muscle. On the other hand, dermatan sulphate is a constituent of the perimysium that is a more static extracellular structure.