Retinol binding protein (RBP), a 21kDa single-chain glycoprotein, is the primary plasma transport protein for retinol (vitamin A1). RBP solubilizes the waterinsoluble retinol and works as a vehicle for transport to the target organs, and it may also protect the bound retinol from oxidative damage in the plasma. The circulating RBP binds to another protein called transthyretin (TTR), a 55-kDa plasma thyroxine binding protein. Such protein-protein complex formation is thought to prevent glomerular filtration of low molecular mass RBP. Serum levels of RBP are useful in the detection of liver disease, protein-calorie malnutrition, and vitamin A deficiencies. In addition, the determination of RBP serum levels has been shown to be important in the mediation of antitumor effects.