Abstract Hemoglobin-coated charcoal has been used as a competitive binder in developing a [131I] triiodothyronine uptake assay. Modifying the procedures of Herbert et al. [J. Lab. Clin. Med.66, 814 (1965)] and Braverman et al. [J. Amer. Med. Ass.199, 469 (1967)] and examining various optimum test parameters, we developed a rapid, simpler technique. Correlation with results of the "Triosorb-131" resin sponge assay (Abbott Laboratories) was 0.909. Inter-run precision for a lyophilized serum control was satisfactory (CV, 2.5%). Advantages of the present method include: rapid equilibration time (10 min), easily prepared and dispensed adsorbent, nonquantitative transfer of final reaction mixture, and a linear response to variation in concentrations of binding protein that is of use in the diagnosis of thyroid dysfunction