1. Thyroxine and tri-iodothyronine are bound by liposomes (bimolecular phospholipid membranes produced by dispersing egg-yolk lecithin in water). At pH 7·4 the `partition coefficient' between water and the phospholipid is 1·2 × 104 for thyroxine and 2·2 × 104 for tri-iodothyronine. The partition coefficient falls off rapidly in more alkaline solutions as the phenolic group on the hormones becomes ionized.