The structural features responsible for the immunogenicity of certain parts of native protein molecules have been of interest to immunochemists and protein chemists for over three decades. Following the early work of Land­ steiner in 1942, which showed that peptide fragments from silk fibroin exhibited an inhibitory activity toward the reaction of the protein with its antibodies, fragments from many other protein systems have been isolated and studied. However, no concerted effort was (or could be) devoted to the elucidation of the complete antigenic structure of a protein. In order for these endeavors to be successful and meaningful, knowledge of both the amino acid sequence and the detailed three-dimensional structure of the protein is necessary. Such information was not available for a protein until early in the 1960s. This and the fact that protein chemistry was not in fact sufficiently developed early in the 1960s to enable the successful completion of the entire antigenic structure of a protein were major contributing factors for the slow progress in this field. Determination of the antigenic structures of proteins therefore posed a chemical challenge of enormous proportions. For these reasons, many investigators diverted their attention to study of the immunochemistry of homo- or mixed amino acid polymers in the hope that the information derived from these systems might prove useful in the understanding of the immunochemistry of proteins.