Immunosuppressive drugs were given to seven patients with polymyositis. The in-vitro activity of peripheral blood lymphocytes had previously been studied in five of these patients with findings suggestive of disturbed immunological processes. Some improvement occurred in five cases, but only in two was the improvement marked and sustained. In this small series of cases, the response to treatment was best in a patient with polymyositis who showed no evidence of involvement of tissues or organs other than muscle and in a second case with subacute polymyositis occurring in association with an unidentified connective tissue disorder. The response was less satisfactory in two patients with dermatomyositis, in two with polymyositis associated with systemic sclerosis, and in one in whom the muscle disorder complicated rheumatoid arthritis. At present such treatment is usually given only in cases which are resistant to, or intolerant of, steroids. The relative values of steroid and immunosuppressive therapy are discussed; a combination of the two in moderate doses may eventually prove to be the best initial treatment for the disorder.