This work shows that tumor promoter agents (TPA) induce the post-translational modification of the human lymphocyte surface CD5 antigen (Tp67) in several cellular types. Treatment of [32P]orthophosphate- and [35S]cysteine-labeled normal and lymphoblastoid T and B cells with active tumor promoters induced the rapid, transitory and dose-dependent appearance of hyperphosphorylated CD5 forms with higher apparent molecular masses. These changes in the electrophoretic mobility of CD5 molecules were independent of RNA and protein synthesis, as well as of differences in neuraminic acid content. The inhibition of the TPA-mediated changes by protein kinase C inhibitors (staurosporine and 1-(5-isoquinolylsulfonyl)-2-methylpiperazine) indicated its protein-kinase-C-mediated nature. Phosphatase digestion of CD5 immunoprecipitates reverted the TPA-mediated mobility changes showing its dependence on phosphorylation. Neuraminidase digestion of intact cells revealed that the target of the TPA effects are surface-expressed CD5 molecules. In conclusion, we suggest that the heterogeneity in the electrophoretic mobility induced by TPA could reflect some structural and/or functional differences within CD5 molecules.