Groups of BALB/c mice were treated with various conjugates of 2,4 dinitrophenyl (DNP) and BALB/c myeloma proteins belonging to the four subclasses of IgG (IgG1, IgG2, IgG2b, IgG3). Immediately therafter, they were challenged with DNP-keyhole limpet hemocyanin in complete Freund's adjuvant and antibody to the hapten was measured by direct and indirect hemolytic plaque assay. The results show that all subclasses of IgG are effective as tolerance-inducing carriers. However, the ability of induce tolerance is dependent upon the concentration of hapten bound to each myeloma protein. Tolerogenic conjugates suppress both direct and indirect plaque-forming cells in all types of antibodies measured (IgG1, IgG2a, IgG2b, IgGa), whereas, non-tolerogenic conjugate failed to suppress them. The intact molecule of IgG but not its fragments (Fab, F(ab)2', Fc) appear necessary as tolerance-inducing carriers. It is suggested that the ability to induce tolerance is related to the capacity of the tolerogenic conjugates to cause receptor blockade.