At least three groups of polypeptides of the inner membrane of rat liver mitochondria have been shown to be exposed to the exterior surface by several techniques: lactoperoxidase-catalyzed iodination of mitochondria and inner membrane/matrix vesicles (mitoplasts), reaction of mitochondria and mitoplasts with the membrane-impermeable diazonium salt of sulfanilic acid, and controlled proteolysis of mitoplasts. These classes of proteins, separated by dodecyl sulfate gel electrophoresis, have polypeptide molecular weights of 73,000, 31,000, and 26,000. In addition, four other groups have been shown to be exposed to the exterior surface by at least one of these techniques: these components have polypeptide molecular weights of 130,000, 87,000, 16,000, and 10,500. A class of proteins, which makes up 50 to 60% of the total mitochondrial protein and which can be easily extracted from mitoplasts by freeze-thaw fractionation or other procedures designed to separate "matrix" protein from "membrane" protein, is shown not to be exposed to the outer surface of the inner membrane by these techniques. This class of proteins contains polypeptides of various molecular weights and includes the major 165,000 molecular weight polypeptide, identified with carbamyl phosphate synthetase.