The comparative immunology and biochemistry of plasma retinol transport were studied using radioimmunoassays previously developed for human and for rat retinol-binding protein (RBP). Serum or plasma from 25 species of verebrates, from the mammalian orders Primates, Artiodactyla, Perissodactyla, Carnivora, and Rodentia and from the classes Aves, Amphibia, and Pisces, were assayed. There was a high degree of immunological specificity within a given mammalian order. Sera from seven subhuman primate species tested reacted in the human RBP immunoassay, and sera from four of five rodents reacted in the rat RBP immunoassay. Primate sera failed to react in the rat RBP immunoassay, and rodent sera failed to react in the human RBP immunoassay. Except for a slight reactivity of canine serum in the human RBP immunoassay, other sera showed no immunoreactivity. Using gel filtration, apparent molecular weights were estimated at 60,000-80,000 for the retinol transport systems in whole serum from cow, swine, chicken, and dog. Canine RBP was isolated and partially characterized. Purified canine RBP was generally similar to human and rat RBP with regard to molecular weight (approximately 20,000) and other properties. In plasma, canine RBP circulates as a protein-protein complex of higher apparent molecular weight. The complex remains to be characterized. These data suggest that mammals in general have a retinol transport system similar to the human and rat transport systems but that immunologically important differences in RBP occur among mammalian orders.