Triiodothyronine (T3) binding to Ficoll-Isopaque purified human lymphocytes was studied. During incubation of lymphocytes with [125I]T3 in a calcium-free medium at 37 degrees C, maximal uptake of T3 in nuclei occurrred after 2 h and declined after prolonged incubationd incubation . Incubation of lymphocytes with T3 concentrations ranging from 1 X 10(-11) TO 1 X 10(-9) mol/l and subsequent treatment with Triton X-100 to strip off [125I]T3 bound with low affinity was used for the estimation of affinity and capacity of nuclear T3 binding sites. The mean equilibrium affinity constant (Ka) estimated with the Scatchard method in 11 euthyroid healthy subjects was 4.5 X 10(9) l/mol, and the mean maximal binding capacity 25 X 10(-5) mol/100 mug DNA. In a female patient with peripheral resistance to thyroid hormone action, the estimated Ka was 3.5 X 10(9) l/mol and the number of T3 binding sites 37 X 10(-15) mol/100 mug DNA. Although not statistically different from the mean value in euthyroid subjects, this Ka value was outside the range of control values observed and was considered presumptive evidence that the nuclear T3 receptors in this patient have abnormally low affinity for its ligand. The nuclear T3 binding capacity in this patient was significantly increased.