The fungal metabolite Cyclosporin-A (CyA) was investigated for its activity on several cell-mediated immune responses. Oral administration of 70 mg kg-1 of CyA for 5 days to C3H/HeN mice completely abolished the in vivo antigen-dependent production of a lymphokine capable of increasing macrophage cytotoxicity against tumor cells. Moreover, spleen cells from CyA treated mice were significantly depressed in their ability to produce in vitro lymphokines in response to PHA, whereas only a slight depression was observed when ConA was employed to induce lymphokine production. In parallel to the depression of proliferation-independent immune response, spleen cells from CyA treated mice showed a strongly depressed proliferative response to PHA, marginal reduction being observed in the response to ConA. B-lymphocytes did not seem to be affected by in vivo treatment with CyA, judging from the proliferative responses to LPS. Macrophage responses also remained unaltered after CyA treatment. No depressions in natural or lymphokine-induced macrophage cytotoxicity and in monokine production were in fact observed in CyA treated mice. Finally, a short-lived depression of natural killer (NK) activity was observed after CyA administration. These results indicate that in vivo CyA treatment selectively depressed cell-mediated functions of lymphocytes of the T-cell lineage. The hypothesis that T-cell lineage. The hypothesis that T-helper lymphocytes are the preferential target of CyA immunodepression is discussed.