The therapeutic effect of goat anti-human thymocyte antiserum globulin (ATG) was assessed in 10 patients with myasthenia gravis. All subjects had far-advanced, debilitating disease poorly responsive to anticholinesterase therapy. Prolonged, low-dose ATG therapy was used, with 1.0 to 2.6 gm ATG protein administered intramuscularly over a 28-to 73-day period. Therapeutic responses of varying degrees were noted in 8 of 10 patients. Completion of a course of ATG treatment and discontinuation of the drug did not lead to acute relapse. Follow-up examinations for over 5 years have been maintained. A mean remission period of approximately 2 years was observed. This therapy deserves further evaluation; subjects with progressive myasthenia gravis despite prior thymectomy may represent ideal candidates.