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 insulin in the plasma sample. Their method, which is so elegantly simple in concept, is made possible by the ability to detect with ease extremely low levels of radioactivity, and by the exquisite specificity of an antibody capable of specifically binding the analyte. Such a combination of sensitivity and specificity is the basis of this versatile analytical tool called radioimmunoassay (RIA). Twelve years later, Engvall and Perlmann (1971) and Van Weemen and Schuurs (1971) independently introduced the use of enzymes as another category of sensitive and even more versatile labels for use in immunoassays. Engvall and Perlmann (l971) coined the term ELISA, which stands for Enzyme Linked Immunosorbent Assay.