Clofazimine (B663) has been the subject of considerable study in the experimental animal because of its antimycobacterial activity. This is the first study of the tissue distribution of B663 in man. Three leprosy patients were studied at autopsy; a skin biopsy was studied in a fourth patient. Tissue concentrations were analyzed by a simple chemical method. Tissue coloration was observed in the internal organs. Initially, the subcutaneous tissues appeared gray to opaque, but within minutes of exposure to the air began turning a yellow-orange color; after some ten minutes, tissues high in fat content were orange-red. Highest concentrations of B663 were observed in tissues with high fat content and in the bile. Tissues with a reticuloendothelial component or high vascularity also showed relatively high concentrations. High levels were present also in the liver and in the gall bladder.