Analyses of the distribution of isotopically labeled antigens in the animal were combined with autoradiography of the tissues. After intravenous injection of heavily substituted S35-sulfanilazo-bovine γ-globulin into mice, we found protein-bound S35 in the cytoplasm of the Kupffer cells of the liver. Similarly, activity of injected biosynthetically labeled S35-RbSA was concentrated in the cytoplasm of lymphoid cells of the spleen, particularly in the red pulp. After injection of lightly labeled S35-sulfanilazo-bo-vine γ-globulin into the foot pads of rabbits, the radioactive material was concentrated in the cytoplasm of the eosinophilic macrophages of the popliteal lymph nodes. In all these cell types, the protein-bound S35 was either present within the cytoplasm or adsorbed to the cellular membrane. Little intranuclear radioactivity was evident.