Following optimization of the reaction conditions, e.g. concentration of oxidizing agents, reaction time, volume of reaction mixture, and pH, chloramine T and the new iodination reagent, Iodogen, were compared for their effectiveness in radioiodination of insulin, glucagon, human growth hormone (hGH), and rabbit anti-mouse IgG. The radioactive peptide hormones prepared were analyzed for the presence of aggregate and breakdown products by polyacrylamide gel electrophoresis (PAGE) at pH 8.9, the rabbit anti-mouse IgG was tested for the presence of low molecular weight damage products by gel filtration on Sephadex G-50. The results demonstrate that with respect to iodine incorporation, specific activity, and immunological reactivity either method can be used to prepare under carefully controlled conditions a wide range of tracers with high specific activity at minimal oxidation damage. These tracers are shown to be highly suitable in radioimmunoassays after previous purification by PAGE and gel filtration, respectively.