The fate of heterologous, isologous and homologous ferritin‐I125 injected into rat footpads was compared by determination of radioactivity in sera and organs, and by radioautography and electron microscopy. The clearance of heterologous ferritin‐I125 from circulation was significantly faster than that of isologous or homologous ferritin‐I125. This was supported by measurement of radioactivity in various organs, and by radioautography and electron microscopy of popliteal lymph nodes which reveled structural details of macrophages undergoing antigen uptake. These observations permit the following conclusions. (1) In most reticuloendothelial cells there is some nonspecific pinocytosis of antigen which is not related to immunogenicity. (2) The induction of massive immunologically specific pinocytosis by macrophages may be due to specific antigen recognition by receptors located at the cell surface. (3) Heterologous ferritin ingested by macrophages are mostly found in vacuoles or scattered in the ground cytoplasm. However, some appear in the nucleoplasm, usually in association with loose strands of chromatin materials. (4) Ferritin molecules are conspicuously absent from mitochondria, the rough‐surfaced endoplasmic reticulum and Golgi apparatus. (5) The fiber‐associated reticular cells of the primary nodule and germinal centers may correspond to “dendritic reticular cells” or “dendritic macrophages,” capable of long‐term retention of antigens.