Plasma protein contamination accounted for high affinity thyroid hormone binding to a nuclear protein extract from human blood mononuclear cells. The degree of contamination was directly related to the number of times the cells were washed before the nuclear protein was extracted. After three washes, no specific binding was detected. Plasma contamination was estimated to be 0.3-0.5 microliter/assay tube. The TBG concentration in this volume of plasma was consistent with the concentration of 0.2 micrograms/ml measured by RIA in the nuclear extract. Total specific binding of [125I] T4 and [125I] T3 can be attributed to plasma contamination, as 0.3 microliter of plasma gave identical binding. Furthermore, a TBG antiserum abolished specific T4 and T3 binding to the nuclear protein extract. These data are consistent with TBG being the major plasma contaminant. In fourteen normal, euthyroid subjects, the higher mean maximum binding capacity (MBC) for T4 (1.63 +/- 0.27 SD) compared to T3 (0.25 +/- 0.21 pmol/mg protein; P less than 0.001) and the same binding affinity (Ka, 2.0 +/- 0.7 x 10(9)/mol) for both T4 and T3 are the same as found in plasma. The higher MBC for T4 (5.8 +/- 1.0) and T3 (0.69 +/- 0.2 pmol/mg protein) in two hypothyroid subjects are consistent with the higher mean TBG concentration of 28 +/- 5 micrograms/ml compared to the level in ten normal subjects (19.2 +/- 1.8 micrograms/ml; P less than 0.001). In two subjects with low normal TBG concentrations of 15 micrograms/ml, the T4 maximum specific binding of 9% was lower than found in normal subjects 16.6 +/- 7.6%. We conclude that the methodology described previously and used in this study is invalid for measuring thyroid hormone nuclear receptor status in humans.