Various cell types from adult rats, including thoracic duct cells, peripheral blood leukocytes, fractions of peritoneal exudate cells rich in mast cells, eosinophils or macrophages (monocytes), were examined for their ability to incorporate C14-amino acid into serum proteins. Autoradiography of immunoelectrophoretic patterns prepared with concentrated culture fluids demonstrated that macrophages, isolated by means of their adherence to glass, were much more active than any other cell type in the production of β1C and transferrin. Peritoneal and lung macrophages from various other species, including mice, rabbits and guinea pigs, invariably produced β1C, and in the case of primates also β1E. The role of macrophages in the production by various tissues of serum proteins associated with the complement system was discussed. Rats and mice were the only species studied in which transferrin was produced by lymphoid tissue and peritoneal macrophages.