Treatment of rats with the dopaminergic ergot alkaloid bromocriptine (BRC) inhibited the following immune reactions: contact sensitivity skin reaction to dinitrochlorobenzene (DNCB); antibody formation to sheep red blood cells and to bacterial lipopolysaccharide; adjuvant arthritis; and experimental allergic encephalitis. Immunosuppressive doses of BRC (5 mg/kg) decreased the serum prolactin (PRL) levels from 84.8 +/- 15.9 ng/ml to 4.9 +/- 1.6 ng/ml. Further studies on DNCB contact sensitivity and on antibody formation revealed that the immunocompetence of BRC-suppressed animals could be restored by additional treatment with either prolactin (PRL) or growth hormone (GH). Treatment with adrenocorticotropic hormone antagonized the restoring effect of PRL and GH. These results suggest that BRC suppressed immunity by its inhibition of PRL, and possibly also by inhibition of GH secretion.