PKCdelta has been implicated in the signalling events after engagement of the antigen specific receptor on B cells and the Fc-epsilon receptor on mast cells. Employing our recently established PKCdelta null mice , we here investigate the physiological function of PKCdelta in CD3+ T cells. As result, administration of anti-CD3 antibodies in vivo induced markedly increased blood plasma IL-2 levels (but not IL-4, IFN-gamma, TNF-alpha and IL-6 levels) in the PKCdelta null mice, when compared to wild-type sibling controls. Additionally, in vitro T cell proliferative responses to allogenic MHC are significantly enhanced in PKCdelta-deficient CD3+ T cells. These findings suggest that PKCdelta serves a so far unrecognized role in TCR-induced negative regulation of IL-2 cytokine production and T cell proliferation.