Anamnesis of the Radioimmunoassay - A Historical Reminiscence: Several requirements have to be met before the first radioimmunoassay of a proteohormone (i.e., insulin) could be developed. One of these requirements was that iodination ought not substantially impair the immunologic properties of the insulin molecule. Since the reversibility of antigen adsorption to antibodies is the most important basis of all immunoassays, the adsorptive behavior of proteins first had to be studied at extremely low protein concentrations. Only after these requirements had been investigated was direct electrophoretic detection of a nonprecipitating anti-hormone antibody possible by desorption of insulin on filter paper. Later, the inaccuracy of the species specificity of insulin-binding antibodies proved to be particularly useful for the determination of insulin in human sera.