The localization of individually specific antigenic determinants on the monoclonal gammaM Mö was studied in gel diffusion experiments with 19S gammaM Mö and its 7S subunit. Various types of such determinants were delineated. Some were demonstrable both on the intact macroglobulin molecule and on the subunit. The latter was often unable to precipitate with the corresponding antibody, but combined with it and inhibited precipitation. Immunization of rabbits with purified gammaMs Mö was the most useful way to obtain antisera demonstrating individually specific antigenic determinants on the subunit by direct precipitation. Still other determinants were apparently destroyed by mild reduction of the gammaM molecule. Reduction and reassociation of mixtures of two monoclonal gammaM-globulins resulted in formation of hybrid molecules containing subunits of different origin. Individually specific antisera against the two components were useful for demonstrating hybrid formation. Serological activity, i.e., cold agglutinin activity, was present in the hybrid molecules.