The cellular distribution of intravenously administered 3H-heme, 125I-hemopexin and 125I-albumin was studied in rabbits. Radioautography of the tissue slides of several organs was examined by light and electron microscopy. All experiments were terminated 60 min after injection of the isotope-labeled materials. 3H-heme and native monomeric 125I-hemopexin (heme saturated) were found to be associated with the endoplasmic reticulum and microbodies of the hepatocytes. Aggregated hemopexin was also taken up by macrophages of lung alveoli, splenic pulp, interstitial cells of the kidney and Kupffer cells. 125I-albumin (heme saturated) could not be located in liver, spleen, bone marrow, lung, or kidney tissues.