The paper by C. N. Hales and P. J. Randle describes an improved method for the immunoassay of insulin using 131I-labeled insulin and insulin antibody. The authors build upon previous methods by Yalow & Berson (1960) and Grodsky & Forsham (1960), which relied on isotope dilution and the separation of free insulin from antibody-bound insulin. The new methods involve precipitating insulin antibody with anti-γ-globulin serum, which allows for greater simplicity, rapidity, and sensitivity compared to earlier techniques. The paper details the experimental procedures, including the preparation of 131I-labeled insulin, insulin antibodies, and anti-γ-globulin serum. It also discusses the theoretical principles of isotope dilution and immunological assays, and provides practical details such as the concentration of reagents and the optimal timing of reagent additions. The authors demonstrate the effectiveness of their methods through various experiments, including the recovery of radioactivity, the effect of different concentrations of unlabelled insulin, and the comparison of methods B and C. The methods are shown to be suitable for assaying insulin in blood plasma, with the sensitivity and accuracy being comparable to those of biological assays. The paper concludes by suggesting that these methods could be applied to the microassay of other proteins that can be purified and labeled with radioactive isotopes.The paper by C. N. Hales and P. J. Randle describes an improved method for the immunoassay of insulin using 131I-labeled insulin and insulin antibody. The authors build upon previous methods by Yalow & Berson (1960) and Grodsky & Forsham (1960), which relied on isotope dilution and the separation of free insulin from antibody-bound insulin. The new methods involve precipitating insulin antibody with anti-γ-globulin serum, which allows for greater simplicity, rapidity, and sensitivity compared to earlier techniques. The paper details the experimental procedures, including the preparation of 131I-labeled insulin, insulin antibodies, and anti-γ-globulin serum. It also discusses the theoretical principles of isotope dilution and immunological assays, and provides practical details such as the concentration of reagents and the optimal timing of reagent additions. The authors demonstrate the effectiveness of their methods through various experiments, including the recovery of radioactivity, the effect of different concentrations of unlabelled insulin, and the comparison of methods B and C. The methods are shown to be suitable for assaying insulin in blood plasma, with the sensitivity and accuracy being comparable to those of biological assays. The paper concludes by suggesting that these methods could be applied to the microassay of other proteins that can be purified and labeled with radioactive isotopes.