The basis for the utilization of synthetic peptides as immunogens was laid during our early studies on the antigenicity of proteins. The initial findings that covalent attachment of tyrosine oligopeptides to gelatin by polymeric techniques resulted in augmentation of its immunogenicity (SELA and ARNON 1960) led us to the synthesis of both linear and branched polymers of amino acids, capable of initiating an immune response (SELA et al. 1962). The availability of these synthetic antigens permitted a systematic elucidation of the molecular basis of antigenicity, including the role of such variables as chemical composition, size, shape, accessibility of epitopes, electrical Charge, optical configuration, and mainly spatial conformation in rendering a molecule immunogenic and in dictating its antigenic specificity (SELA 1969).