Multiple indices of thyroid hormone binding have been studied in sera obtained from patients with thyrotoxic Graves' disease, before and after treatment, and in sera from a group of carefully matched normal controls. Specimens from patients with thyrotoxicosis displayed a decrease in the thyroxine (T(4))-binding capacities of T(4)-binding globulin (TBG) and T(4)-binding prealbumin (TBPA), an increase in serum protein-bound iodine (PBI), and an increase in both the proportion and absolute concentration of free T(4). In addition, a smaller than normal proportion of (131)I-labeled T(3) was associated with TBG during filter paper electrophoresis. After treatment of thyroxicosis, the only residual abnormality detected was a very slight persistent decrease in the T(4)-binding capacity of TBPA, which did not appear to influence the overall thyroid hormone-plasma protein interaction significantly, regardless of whether this was assessed under basal conditions or after enrichment of specimens with stable T(4). It is concluded that the persistent abnormalities in the peripheral metabolism of T(4), previously reported to occur in some patients with treated Graves' disease, probably do not stem from residual abnormalities in the transport of T(4) in the plasma but must arise from abnormalities in T(4) accumulation or metabolism within the tissues themselves.