Lymphocytes were labelled by incubation with [32P]Pi and their plasma membranes isolated. Analysis by one-dimensional and two-dimensional gel electrophoresis revealed a small number of strongly phosphorylated polypeptides. Two of these were especially prominent; they had molecular weights of about 52000 and 90000, were acidic and were apparently not glycosylated. Similar patterns were obtained for quiescent T- and B-lymphocytes from different species and for cultured lymphoblastoid cells, although the relative amounts of the labelled polypeptides varied. Immunoprecipitation analyses of the detergent-solubilized 32P-labelled plasma membranes indicated that the glycosylated polypeptide of the human major transplantation (HLA-A and HLA-B) antigens and its mouse and pig counterparts are phosphorylated. In contrast, no phosphorylation of the membrane-associated immunoglobulin, the mouse Thy-1 antigen or the human HLA-DRw(Ia) antigen was detected. The phosphorylation patterns of human peripheral blood and nude-mouse spleen lymphocytes did not change during the period 5-30min after mitogen stimulation. Therefore a change in the phosphorylation of plasma-membrane protein(s) is probably not an early biochemical event in the initiation of T-lymphocyte and B-lymphocyte growth, although a rapid transient change cannot be ruled out. Similar plasma-membrane phosphorylation patterns were also obtained by incubating the purified plasma membrane with [gamma-32P]ATP. The phosphorylation of the 90000-mol.wt. polypeptide was particularly rapid and was stimulated by the addition of cyclic AMP.