In addition to the previously recognised thyroid hormone-binding proteins, three plasma components possibly involved in T3 and T4 transport have been identified in chicken plasma using gel filtration at pH 7.4. In mature female, though not male and immature female birds, a significant amount of T3 (11%) is bound to very-low- and low-density lipoproteins. These could mediate transport of the hormone into the egg. Similarly vitellogenin appears to bind small quantities of both T3 and T4. Selective precipitation of the lipoproteins and vitellogenin from laying-hen plasma produces a binding profile identical to that observed in male and juvenile female birds. Treatment of adult male birds with oestrogen induces a binding profile for T3 similar to that observed in adult females. Also a low-molecular-mass protein (9 X 10(3) D) which preferentially binds thyroxine in substantial amounts (20-30%) was found in plasma from adult male and female and immature female birds. The technique employed allows detection of plasma thyroid hormone-binding proteins which have not previously been resolved by simple electrophoretic procedures.