T. T. Ngo and H. M. Lenhoff Department of Developmental and Cell Biology University of California, Irvine, CA 92717 In 1959, Yalow and Berson used insulin labeled with radioactive iodine to develop a quantitative immunological method for determining the amount of insulin in human plasma. Their method depends upon competition between insulin labeled with radioactive iodine (II 1) and unlabeled insulin from plasma for a fixed and limited number of specific binding sites on the antibody to insulin. The amount of the labeled insulin bound to the antibody is inversely proportional to the amount of...