The concentration of thyroxine in bile is about the same as its concentration in plasma (Osorio & Myant, 1963). In the plasma, however, only about 0·1% is present in free solution; the remainder is bound to specific proteins. In an analogous way most of the 'thyroxine' in bile is conjugated with glucuronic acid and only about 5% is present in a chemically free form (Osorio & Myant, 1963). None the less, if all of this thyroxine were present in simple physical solution there would be a gradient in free thyroxine between the bile and the plasma of about 50:1. This is unlikely since cell membranes are fairly permeable to the hormone (Hillier, 1970) and no example of active thyroxine transport has yet been demonstrated. In aqueous solution, bile salts form micelles (Hofmann, 1968). Each micelle is composed of 20–25 molecules arranged in a sphere with the hydrophobic portion of the