Electrophoretic studies employing a variety of media and buffer systems have repeatedly shown that tracer thyroxine (T4) moves with two distinct protein bands in sheep serum. These bands correspond to thyroxine-binding globulin (TBG) and serum albumin (Annison, 1960; Farer, Robbins, Blumberg & Rail, 1962; Refetoff, Robin & Fang, 1970). It has been pointed out by Gordon & Coutsoftides (1969) that such electrophoretic techniques are unlikely to depict closely T4 binding in vivo. For this reason we have developed a competitive-binding technique, using Sephadex G-25, which has enabled the measurement of the T4-binding properties of sheep serum proteins at physiological pH. This technique is similar in principle to that previously described by Pearlman & Crépy (1967).
Sephadex G-25 binds T4 in a highly predictable way. When a constant amount of G-25 is in contact with a constant volume of buffer, at equilibrium, the T4 present