We investigated whether the immunosuppressive effect of bromocriptine in mice is due to its direct effect on lymphocyte functions or through inhibition of prolactin secretion. Incubation of mouse Babl/c splenic lymphocytes with bromocriptine in vitro at a concentration of around 0.5 to 1 μg/mL causes an inhibition of antigen- or mitogen-induced proliferation. However, bromocriptine in vitro has no effect on lymphokine production (γ-interferon and interleukin-2), expression of interleukin-2 receptor or lymphocyte cytotoxic function. Furthermore, treatment of Babl/c mice with bromocriptine inhibits the mixed lymphocyte reaction and mitogen stimulation, as well as primary and secondary antibody production. However, we postulated that the inhibition of ex vivo functional activity could not account for a direct cytostatic or cytotoxic effect of bromocriptine. This is supported by the in vitro data, which shows that bromocriptine has no effect on proliferating P-815 mastocytoma tumor cells. Finally, (NZB/NZW) F1 mice spontaneously develop a disease similar to systemic lupus crythematosus. In both non-autoimmune Babl/c mice and (NZB/NZW) F1 lupus-mice, the serum level of bromocriptine achieved by a treatment with 5 mg/kg on average is 2–6 ng/mL. On the one hand, this dose is sufficient to significantly alter the ex vivo functional tests in Babl/c mice and to show a beneficial effect in the in vivo model of female lupus-mice. On the other hand, the lowest concentration which could have an inhibitory effect on antigen- and mitogen-induced proliferation in vitro is 200 ng/mL, ie 50 times more than that required in vivo to obtain significant reductions of proteinurea, glomerular membrane proliferation and immune deposits in lupus-mice. The serum levels of γ-interferon and interleukin-2 are reduced in lupus-mice when compared with Babl/c mice. The treatment with bromocriptine does not influence these parameters. In conclusion, our data demonstrate that the major immunosuppressive activity of bromocriptine is probably dependent on its hypoprolactinemic effect.