Protein kinase Cdelta (PKCdelta) inhibits proliferation and decreases expression of the differentiation marker glutamine synthetase (GS) in C6 glioma cells. Here, we report that distinct, specific tyrosine residues on PKCdelta are involved in these two responses. Transfection of cells with PKCdelta mutated at tyrosine 155 to phenylalanine caused enhanced proliferation in response to 12-phorbol 12-myristate 13-acetate, whereas GS expression resembled that for the PKCdelta wild-type transfectant. Conversely, transfection with PKCdelta mutated at tyrosine 187 to phenylalanine resulted in increased expression of GS, whereas the rate of proliferation resembled that of the PKCdelta wild-type transfectant. The tyrosine phosphorylation of PKCdelta and the decrease in GS expression induced by platelet-derived growth factor (PDGF) were abolished by the Src kinase inhibitors PP1 and PP2. In response to PDGF, Fyn associated with PKCdelta via tyrosine 187. Finally, overexpression of dominant negative Fyn abrogated the decrease in GS expression and reduced the tyrosine phosphorylation of PKCdelta induced by PDGF. We conclude that the tyrosine phosphorylation of PKCdelta and its association with tyrosine kinases may be an important point of divergence in PKC signaling.