When labeled to monoclonal antibodies (MAbs) or their fragments, 123I can be used for imaging or for predicting the treatment potential and radiation dosimetry of 131I labeled to the same molecular species. Because 123I (p,5n) from the Crocker Nuclear Laboratory is in dilute solution, when compared with commercial 125I of labeling grade, we have evaluated labeling parameters using Chloramine-T as the oxidant and derived an optimum set of labeling conditions that provide a 60-80% radiochemical yield of highly immunoreactive antibody. When Lym-1, an IgG-2a murine antibody against human lymphoma, was used, yields of labeled immunoglobulin were decreased by protein or Chloramine-T concentrations less than 0.4 microgram/microliter and 0.8 microgram/microliter, respectively; denaturation of the immunoglobulin occurred when the Chloramine-T concentration was greater than 1.0 microgram/microliter. Optimum labeling occurred at pH 7-8 with deleterious effects when the pH was below 5 or above 10. An optimum method for labeling antibodies with multimillicurie amounts of 123I (less than one iodine atom per 100 antibody molecules) is described. Some of the observations derived from this study are also applicable to the preparation of treatment doses of 131I-labeled antibodies, wherein the amount of antibody can be a restrictive factor.