Summary: Protein synthesis by leukocytes from human peripheral blood was studied by measuring the incorporation of 14C amino acid into protein in vitro. The cell suspensions were rich in lymphocytes but contained other types of cell from the blood. Analysis of culture supernatants by DEAE column chromatography and starch gel electrophoresis showed that radioactive substrate was incorporated into a broad range of serum proteins. Autoradiography of immunoelectrophoretic patterns showed that the radioactivity was contained in several specific proteins. IgG and IgM were labelled, but these bands were less strongly labelled than some of the other fractions. It is not possible to determine from these data what contribution was made by granulocytes and platelets to the total protein synthesized. It was calculated that 108 lymphocytes synthesized 30 µg IgG globulin in 48 hr.