Transthyretin (TTR), formerly called prealbumin, one of the transporters of the hormone thyroxine and retinol-binding protein (RBP), the specific carrier of vitamin A, forms, under physiological conditions, a macromolecular complex that prevents glomerular filtration of the low-molecular-weight RBP in the kidneys. This paper describes briefly the three-dimensional structure of the two proteins as determined by X-ray diffraction analysis of single crystals and that of the complex that the two molecules form in plasma and discusses the non-crystallographic evidence that supports the model of the macromolecular complex.