Rapamycin (RPM) is a macrolide fermentation product that prolongs rodent allograft survival more potently and effectively than cyclosporin A (CsA) and FK506. Experiments in vitro have shown that RPM inhibits lymphoproliferation by mechanisms of action that are different from other immunosuppressants. Much less is known, however, about the effects of RPM on immune cells in vivo compared to other immunosuppressive drugs. Others have shown that in vivo treatment with CsA suppresses the responsiveness of cells in the mixed lymphocyte response (MLR). Therefore, to investigate the effects of RPM in vivo, rats were treated with RPM and their lymphoid cells used as responder cells in the MLR. We confirmed that the proliferation of cells in the MLR was decreased after treatment with CsA in vivo. In contrast, treatment with RPM in vivo greatly increased the proliferative response to alloantigen in the MLR. These findings show that the effects of RPM and CsA on immune cells in vivo differ. Perhaps the cells proliferating in the MLR after in vivo RPM treatment play a role in the regulation of the immune system that enables this immunosuppressant to prolong allograft survival so effectively in rodents.