Rapamycin, a new antifungal antibiotic, was found to inhibit the immune response in rats. It totally prevented the development of two experimental immunopathies (experimental allergic encephalomyelitis (EAE) and adjuvant arthritis (AA)) and the formation of humoral (IgE-like) antibody. It was about half as potent as cyclophosphamide in inhibiting EAE. In AA and on antibody formation, rapamycin and cyclophosphamide were about equipotent, whereas methotrexate was more potent. The immunosuppressant activity of rapamycin appears to be related to inhibition of the lymphatic system.