Summary: Immunization with each of 8 homogeneous antigens led to the formation of multiple antibodies. This is attributed to a different degree of adjustment of the antibody-molecules to particular factors (sub-groups) within the determinant group of the antigen. The bonding force of determinant arsenic-acid groups is stronger than that of sulfonic-acid groups, probably because of their higher ionic strength. A positively charged azoprotein with quaternary ammonium groups proved to be just as powerful an antigen as arsanil-azoproteins. The determinant group of substituted proteins (azoproteins, iodoproteins) includes also a portion of the protein-molecule. We distinguish 3 types of antibodies: a) low-grade, b) imperfect and c) special antibodies. Imperfect antibodies are precipitated only by the homologous antigen, special antibodies by the homologous as well as by heterologous antigens containing the adequate determinant factors. A rational nomenclature for the multiple antibodies of an immune serum is proposed. We designate each antibody by the determinant factors indispensable for its precipitation. The amount of different antibodies in two immune sera to azoproteins has been determined (table 4 and 5). The essential reasons for the attachment of antibody to antigen are briefly discussed.