Loss of radiolabel after in vivo administration of labeled monoclonal antibodies (MAbs) to cancer patients is a likely cause of the low levels of tumor uptake of MAb which have been observed. In this study, we have evaluated the utility of N-succinimidyl 3-(tri-n-butylstannyl)benzoate (ATE) for the radioiodination of 81C6, a MAb reactive with the extracellular matrix antigen tenascin associated with gliomas and other tumors. In vitro binding properties of MAb labeled via ATE were slightly better than those of the Iodogen preparations. Paired-label studies were performed in athymic mice bearing s.c. D-54 MG xenografts and injected with both 81C6 labeled with 125I using the ATE method and 131I using the Iodogen method. These studies demonstrated that use of the ATE method (a) decreased thyroid uptake by 40- to 100-fold, suggesting a lower rate of dehalogenation compared to MAb labeled using Iodogen; (b) increased tumor uptake by as much as a factor of 4 at Day 1 to more than 12-fold at Day 8; and (c) resulted in superior tumor-to-normal-tissue dose ratios. The specificity of MAb uptake was investigated in a paired-labeled study comparing the distribution of 81C6 and isotype-matched control 45.6, both labeled using the ATE procedure. Localization indices for tumor ranged between 6 at Day 1 to 34 at Day 7, values considerably higher than those reported previously for 81C6 and 45.6 radioiodinated using a conventional method (chloramine T). These results demonstrate that the ATE method may be a valuable approach for labeling MAbs with iodine nuclides.