Protein kinase activities and cyclic AMP binding capacity were investigated in human peripheral blood cells from leukemic patients and normal controls. Using [gamma 32P] ATP as phosphoryldonor, the phosphorylating activities were not found to be significantly different in either normal or leukemic cells when measured on both artificial basic and acidic substrates. In contrast, the GTP-dependent casein kinase activity, CK2, which is almost undetectable in normal granulocytes, was markedly increased in highly proliferating myeloblastic cells from patients with acute myelogenous leukemia (AML) or with chronic myelogenous leukemia in blastic crisis (BC-CML). Levels of endogenous phosphotyrosine were not higher in leukemic cells than in normal peripheral lymphocytes or granulocytes. Finally, cAMP binding capacity was found to be increased in several types of proliferating leukemic cells, due to a higher amount of the R1-type regulatory subunit of the cAMP-dependent protein kinases. Specific patterns of cAMP binding proteins observed in the different types of normal blood cells were rather blurred in leukemic cells. In conclusion, modifications observed in human leukemic cells seem to be more related to proliferation or blockage in normal differentiation than to their cellular origin.