Platelet-derived growth factor BB (PDGF) stimulates DNA synthesis through a mechanism that is at least partially dependent upon Src family tyrosine kinases, although the signal transduction pathway downstream of Src is poorly understood. We have studied the signaling between Src and different protein kinase C (PKC) isoforms and its possible role in the regulation of PDGF-stimulated DNA synthesis. We found that Src promoted the tyrosine phosphorylation of PKCdelta, and its subsequent degradation. Enforced expression of PKCdelta inhibited PDGF-stimulated DNA synthesis, whereas expression of PKCalpha and PKCepsilon did not, a finding consistent with a model in which PKCdelta negatively regulates G1-to-S-phase progression. We used mutagenesis to map a critical Src phosphorylation site on PKCdelta to tyrosine 311. A mutant form of PKCdelta in which tyrosine 311 was replaced with phenylalanine (Y311F) was more stable in the presence of Src, suggesting that Src-induced degradation was a direct result of PKCdelta tyrosine phosphorylation. We conclude that PKCdelta is downstream of Src but is unlikely to play a positive role in the signaling pathway by which Src promotes DNA synthesis.