AVARIETY of extracellular binding proteins enhance the solubility of steroid hormones in biological fluids and facilitate their transport from steroidogenic tissues to target cells (1). In mammals, albumin is quantitatively the most important of these proteins and provides a large reservoir of loosely bound hormone that may be released within some target tissues (2). Two albumin-related proteins, vitamin D binding protein and α-fetoprotein, also appear to have arisen by duplication of a common ancestral gene (3) and are characterized by higher affinities and specificities for steroid ligands. The molecular composition and structure of the vitamin D binding protein have already been described in detail (4), and this review will concentrate on the molecular properties of two other biologically important classes of extracellular steroid binding proteins: corticosteroid binding globulin (CBG) and the sex-steroid binding proteins; sex hormone binding globulin (SHBG) and androgen binding protein (ABP). These proteins are far less abundant than those of the albumin superfamily, but they bind biologically active steroid hormones with remarkably high affinity and specificity. As a result, they not only transport steroid hormones but also modulate steroid bioavailability (5), and may participate directly in the delivery of steroids to certain cells by interaction with plasma membrane receptors (6). Indeed, it is becoming increasingly obvious that their activities may be attributed to unique structural properties; a greater understanding of which may help define their physiological importance.