Ethanol has been reported to be immunosuppressive. We have studied the effects of ethanol on early activation events related to the proliferative response of human T lymphocytes. Ethanol inhibited T-cell proliferation in a dose-dependent manner. To define the target of this ethanol-mediated inhibition of T-cell function we examined its effect on the activation of T lymphocytes or induction of competence (acquisition of responsiveness to interleukin (IL)-2 or IL-4) by phytohemagglutinin (PHA) or the combination of phorbol dibutyrate (PDB)/ionomycin. Ethanol inhibited induction of competence with PHA by up to 50% when compared to control cells. In contrast to the effects on PHA-mediated activation of the cells, ethanol exerted no inhibitory action on the induction of competence by PDB/ionomycin. Ethanol also inhibited the induction of c-fos by PHA but not by PDB/ionomycin. To investigate the basis for these differences, the effects of ethanol on Ca2+ mobilization were examined. Ethanol inhibited PHA-induced Ca2+ mobilization in a dose-dependent manner. This inhibition was exerted mainly on transmembrane Ca2+ influx rather than on release of Ca2+ from intracellular stores. Ethanol did not affect Ca2+ mobilization induced by ionomycin. Co-incubation of ionomycin with PHA, during the induction of competence, abolished the inhibition exerted by ethanol when compared to cells treated with PHA alone. The inability of ethanol to exert complete inhibition on cell proliferation may be due to the activation of Ca(2+)-independent pathways by PHA, since combined treatment with ethanol and the intracellular Ca2+ chelator, BAPTA, did not completely inhibit the proliferative response. The inhibitory effects of ethanol on PHA-induced Ca2+ mobilization and subsequent induction of c-fos indicate that ethanol interferes with Ca(2+)-dependent pathways activated by PHA and this may provide the basis for its immunosuppressive action.