Immunohistochemical procedures were used to analyse the subpopulations of mononuclear cells in muscle biopsies from 24 patients with polymyositis. The character of the cellular infiltrate was similar at the perivascular, perimysial, and endomysial sites, with cytotoxic-suppressor T lymphocytes (T8+) and macrophages being the dominant elements. Helper T lymphocytes (T4+) and B lymphocytes were present in smaller numbers. A control series of 17 muscle biopsies from normal subjects and patients with non-inflammatory myopathies and neurogenic conditions was also studied: the numbers of mononuclear cells present were much smaller than in polymyositis, but the ratio of T4:T8 lymphocytes was similar to that found in biopsies affected by polymyositis. We conclude that both cytotoxic-suppressor T lymphocytes and macrophages are important in the pathogenesis of inflammatory myopathy.