T cells express multiple isotypes of protein kinase C (PKC) and although it is well accepted that PKCs have an important role in T cell activation, little is known about the function of individual PKC isotypes. To address this issue, mutationally active PKC-α, -ε, or -ζ have been transfected into T cells and the consequences for T cell activation determined. p21ras plays an essential role in T cell activation. Accordingly, the effects of the constitutively active PKCs were compared to the effects of mutationally activated p21ras. The data indicate that PKC-ε and, to a lesser extent PKC-α but not -ζ, can regulate the transcription factors AP-1 and nuclear factor of activated T cells (NF-AT-1). The ability of PKC-ε to induce transactivation of NF-AT-1 and AP-1 was similar to the stimulatory effect of a constitutively activated p21ras. PKC-ε, but not PKC-α nor activated p21ras, was able to induce NF-KB activity. Phorbol esters induce expression of CD69 whereas none of the activated PKC isotypes tested were able to have this effect. Activated Src and p21ras were able to induce CD69 expression. These results indicate selective functions for different PKC isotypes in T cells. Moreover, the data comparing the effects of activated Ras and PKC mutants suggest that PKC-α, p21ras, and PKC-ε are not positioned linearly on a single signal transduction pathway.