Twenty-three patients with chronic progressive multiple sclerosis (MS), resistant to steroid therapy, were treated with antilymphocyte globulin (ALG) and prednisone (30 mg by mouth daily). Twelve patients completed a full course of therapy (2 ml ALG i.m. daily for 2 weeks and 2 ml i.m. on alternate days for 2 weeks) and 6 others completed at least 2 weeks of daily injections. Six patients experienced an overall improvement of at least 15% using a comprehensive neurological scoring system. Three other patients had limited, but functionally useful improvement in specific neurologic functions. Six months after the completion of therapy, no patient had deteriorated to a level of function below that noted prior to treatment. Adverse reactions, which often necessitated stopping treatment, included fever, local inflammatory reactions, local rash, general malaise, mild anaphylactoid reactions, and enlargement and tenderness of regional lymph nodes. Because of the short duration of immunosuppression and the toxic side effects of ALG, we do not feel that ALG treatment yet deserves to be intorduced as a standard treatment in clinical practice. However, the improvement or arrest of progression seen in these patients who were deteriorating progressively despite steroid therapy would seem to justify a continued search for a safer method of suppressing immunity in MS patients.