Interleukin-12 (IL-12) is an important cytokine in the control of cell-mediated immunity. We have previously shown that endogenous IL-12 plays a role in the development of human allogeneic response. In the present study, we investigated the relationship between Cyclosporin A (CsA)-inhibitory effect and IL-12 pathway during human alloreaction in vitro. CsA addition at the sensitizing phase of primary mixed lymphocyte reaction (MLR) resulted in the inhibition of both p40 and p70 IL-12 production in a dose-dependent manner. In contrast, CsA had no effect on IL-12-receptor beta 1 chain (IL-12 R beta 1) expression in T cells induced upon allogeneic activation. Addition of exogenous IL-12 significantly restored CsA-inhibited alloreactive cytotoxic T lymphocyte (CTL) generation and had a marginal effect on T cell proliferative response. The IL-12-induced restoration of CTL generation was IFN gamma-mediated, as it was significantly altered when anti-IFN gamma was added. The restoration of CTL activity by exogenous IL-12 correlated with the capacity of this cytokine to partially restore granzyme B mRNA expression in alloreactive CTL. This study indicates that inhibition of IL-12 production is a novel additional mechanism for the inhibitory effect of CsA on the development of human allogeneic cytotoxic response.