An analysis was made of the lymphocyte subpopulations in the muscle lesions and the peripheral blood of 25 patients with inflammatory myopathy, in the acute or chronic phase of the disease. Percentages of activated T lymphocytes (65% +/- 3.4), both helper and suppressor/cytotoxic, macrophages (25% +/- 3.2) and B cells (11% +/- 0.9) in the tissues were similar at all stages of the illness; T cells were, however, more common in acute polymyositis than in acute dermatomyositis, where B cells were significantly increased. A loss of circulating OKT8-positive lymphocytes in the peripheral blood was demonstrated, supporting other evidence of disturbed immunoregulation. It was concluded that the attack on muscle fibres is mediated by T cells, macrophages, and B cells, with the first two playing the major roles.