Abstract Biopsy of muscle from 13 cases of human polymyositis permitted an ultrastructural study in which attention was directed to the cellular population of the inflammatory reaction. The constant feature was the presence of small and large lymphocytes, and cells of the latter type were frequently in intimate contact with the surface of the muscle fibres. Macrophages were seen both within and outside degenerating muscle fibres. Plasma cells were seen in 2 cases. Filaments bearing some resemblance to virus inclusions were seen in one case. The implications of the cellular reaction in human polymyositis are discussed and the literature of ultrastructural observations on the muscle in this disease is briefly reviewed.