The article describes the development of an immunoassay method for measuring endogenous plasma insulin in humans. The method involves the use of guinea pig antibodies against beef insulin and the radioisotope iodine-131 (I-131) labeled beef insulin. The immunoassay is based on the competitive inhibition of the binding of labeled insulin to antibodies by human insulin. This technique allows for the detection of very low concentrations of insulin in human plasma, down to a fraction of a microunit. The study outlines the preparation of I-131 labeled insulin, the immunization of guinea pigs with beef insulin, and the procedures for determining insulin concentrations in various subjects, including nondiabetic and early diabetic patients, as well as those with islet cell tumors or leucine-sensitive hypoglycemia. The results show that plasma insulin concentrations vary between individuals and are influenced by factors such as glucose loading and the presence of certain medical conditions. The immunoassay method was found to be more sensitive and specific than previously used methods for measuring insulin in plasma. It was also shown that the method is capable of detecting insulin concentrations in the range of 50 to 100-fold, which is typical for human plasma. The study also highlights the importance of considering factors such as plasma dilution and the potential for interference from other substances in the plasma when measuring insulin levels. The findings suggest that the immunoassay method is a reliable and effective way to measure endogenous plasma insulin in humans, providing valuable information for the diagnosis and management of diabetes and other conditions related to insulin function. The study concludes that the immunoassay method is a significant advancement in the field of insulin measurement and has important implications for the understanding of insulin-related diseases.The article describes the development of an immunoassay method for measuring endogenous plasma insulin in humans. The method involves the use of guinea pig antibodies against beef insulin and the radioisotope iodine-131 (I-131) labeled beef insulin. The immunoassay is based on the competitive inhibition of the binding of labeled insulin to antibodies by human insulin. This technique allows for the detection of very low concentrations of insulin in human plasma, down to a fraction of a microunit. The study outlines the preparation of I-131 labeled insulin, the immunization of guinea pigs with beef insulin, and the procedures for determining insulin concentrations in various subjects, including nondiabetic and early diabetic patients, as well as those with islet cell tumors or leucine-sensitive hypoglycemia. The results show that plasma insulin concentrations vary between individuals and are influenced by factors such as glucose loading and the presence of certain medical conditions. The immunoassay method was found to be more sensitive and specific than previously used methods for measuring insulin in plasma. It was also shown that the method is capable of detecting insulin concentrations in the range of 50 to 100-fold, which is typical for human plasma. The study also highlights the importance of considering factors such as plasma dilution and the potential for interference from other substances in the plasma when measuring insulin levels. The findings suggest that the immunoassay method is a reliable and effective way to measure endogenous plasma insulin in humans, providing valuable information for the diagnosis and management of diabetes and other conditions related to insulin function. The study concludes that the immunoassay method is a significant advancement in the field of insulin measurement and has important implications for the understanding of insulin-related diseases.