Introduction The presence of corticosteroid-binding globulin (CBG) in plasma was detected about 30 yr ago, and of sex hormone-binding globulin (SHBG) about 20 yr ago. In the interval since their discovery, sophisticated binding and RIAs have been devised for their measurement; their concentrations in plasma and other biological fluids, from milk to saliva, have been measured in myriad clinical conditions in humans, and numerous experimental ones in animals; their phyletic distribution has been examined; they have been isolated and their complementary DNAs (cDNAs) cloned; their binding specificities for steroids have been examined in heroic detail; hormonal and drug influences on their plasma concentration in vivo, and their secretion by liver cells in vitro, have been widely explored; they have been the subject of two long, splendid monographs (1, 2) and many shorter reviews (3–11) [wherein the original citations for the first part of this sentence may be found]—and yet we still have not come to a consensus on what it is that they do. Undeniably, they bind certain steroids. But to what biological end?