Phosphorylation of human blood serum proteins by endogenous kinases shows biphasic kinetics. During the early phase mostly beta-globulins are phosphorylated, while during the late phase alpha-globulins are also phosphorylated. In SDS-polyacrylamide gel electrophorograms, 32P-labelled peptides of 6, 14.3, 20, 23, 34, 37 and 40 kilodaltons were detected. No differences in phosphorylation pattern were observed in the sera of patients with various pathological conditions, under the experimental conditions employed.