A procedure is described for the radioiodination of proteins using an iodinated derivative of N-succinimidyl 3-(tri-n-butylstannyl) benzoate (ATE). Adequate removal of unreacted ATE from [125I]ATE was necessary for optimal protein radioiodination. Labeling efficiencies of greater than 60% could be obtained after a 20 min incubation of goat IgG with [125I]ATE at 4 degrees C. Paired-label experiments with goat IgG labeled with 125I using ATE and 131I using Iodogen demonstrated that use of the ATE reagent for protein labeling significantly reduced (P less than 0.005) the thyroid uptake of radioiodine.