The CDw50 antigen is a constitutively non-phosphorylated leukocyte surface molecule which becomes highly phosphorylated in all the normal and lymphoblastoid cells analyzed (peripheral blood mononuclear cells, Molt 4, CEM, 8402, Namalwa), after stimulation with tumor promoter agents (phorbol 12-myristate 13-acetate, phorbol 12,13-dibutyrate, mezerein). This phosphorylation is rapid (within 1-5 min), dose-dependent and results in the incorporation of PO(3-)4 groups on serine residues. Furthermore, the level of CDw50 phosphorylation induced by tumor promoter agents is decreased by the protein kinase C inhibitors staurosporine and 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine. Activation of peripheral lymphocytes with concanavalin A, phytohemagglutinin and cross-linking of CD3 molecules also induces CDw50 phosphorylation, but the response is delayed and less intense than when tumor promoting agents are used. Treatment with any of the aforementioned agents is not accompanied by quantitative changes in the CDw50 surface expression. We therefore conclude that protein-kinase-C-mediated mechanisms are involved in phosphorylation, but not in regulation of the surface expression of the CDw50 leukocyte antigen.