Abstract Objective: To investigate the regulatory effect of PAF on early events in signal transduction during T-cell activation. Methods:T-cell activation was achieved by stimulation of human T cells with CD3 mAb or PMA/ionomycin in the presence or absence of PAF. T cell Pro-liferatilon was determined by 3 H-thymidine incorporation, IL-2 production was measured by MTT method, IL-2R(CD25)expression was evaluatedby flow cytometry and,PKC activity was assayed by the method described by Hauschildt. Results:PAF inhibited CD3 mAb-induced T-cell pro-liferation, IL-2 production and CD25 expression. AIthough it inhibited T-cell proliferation and IL-2 production induced by PMA/ionomycin, PAFfailed to inhibit IL-2 production induced by PMA/ionomycin.The translocation of PKC was also inhibited by PAF if T cells were activated byCD3mAb. Conclusion: PAF inhibits T-cell activation via its inhibitory effect on PKC activation. Its differential effect on IL-2 production suggeststhat PAF regulatory more early events in signal transduction such as the activation of phospholipase C ?-1.