The neuropathies associated with infectious processes, including leprosy, retroviral infections and Chagas' disease, represent the largest group of neuropathies in the world. Segmental demyelination and axonal degeneration of nerve fibres are associated with inflammatory infiltrates which contain a large number of mononuclear phagocytes. In order to learn more about the role played by macrophage activation in the nerve lesions observed in inflammatory neuropathies, we have performed a morphological study of nerves injected with products of activation of macrophages including proteolytic enzymes and cytokines (tumour necrosis factor and alpha beta-interferon). We have also studied the effects on nerve fibres of macrophages activated by ingestion of proteose-peptone, a foreign protein, and in the course of a delayed-type hypersensitivity (DTH) reaction. We have found that proteases and urokinase were potent demyelinating agents and that activated macrophages were also able to induce significant demyelination of neighbouring fibres. In contrast, injection of TNF alpha induced more severe nerve lesions consisting of axonal degeneration of the majority of nerve fibres. We thus conclude that infected macrophages which penetrate the endoneurium and macrophages activated in a DTH reaction can both cause neuropathy.