Six different monoclonal IgG antibodies with specificities for sheep erythrocytes (SRBC) were tested for immunosuppressive ability. Four of them, one IgG3, two IgG2a, and one IgG1, could yield suppression of more than 90% of the anti-SRBC response. The remaining two antibodies, which were both IgG2a, were found to have no significant effect. The degree of suppression correlated well with the amount of antibodies used that could bind to SRBC, as measured by an ELISA assay. High avidity for SRBC was also a factor making the monoclonal antibody more efficient as an immunosuppressor. The response against antigenic determinants on the SRBC other than those for which the monoclonals were specific, was suppressed to an equal degree. This was established by immunizing mice with SRBC using monoclonal anti-SRBC antibodies that did or did not bind to goat RBC (GRBC). The PFC responses against both SRBC and GRBC were then measured. The anti-SRBC and GRBC responses were suppressed in parallel regardless of whether or not the monoclonal reacted with GRBC. None of the tested antibodies displayed any significant ability to enhance the anti-SRBC response. Thus, IgG1 is not the only murine isotype that can efficiently suppress the immune response against SRBC, but IgG2a and IgG3 can also exert this capacity. The mechanism of IgG-mediated suppression is not one of merely blocking single epitopes but involves the immunogenicity of the entire SRBC.