In chickens, erythrocyte isoantigens have hapten-carrier relationships. Specific anticarrier antibody depresses the immune response to the carrier and enhances the immune response to the hapten. Antigenic determinants of "haptenic" isoantigens behave as carriers if they are coated with specific antibody. It is postulated that every humoral antibody response involves the cooperation of a carrier with a hapten and the progressive conversion by antibody of haptens to carriers. Thus a carrier is viewed as an antigenic determinant which is coated with antibody. The antibody-forming cell only synthesizes antibody to the uncoated haptenic determinants. The consequences of this interpretation for the development of immunological maturity and the secondary immune response are discussed.