Nine monoclonal IgG-anti-TNP antibodies were investigated for their ability to modulate anticarrier responses in mice immunized with sheep red blood cells-2,4,6-trinitrophenyl (SRBC-TNP) or keyhole limpet haemocyanin-TNP (KLH-TNP). The antibodies enhanced the anticarrier response when KLH-TNP was used as antigen but suppressed it when SRBC-TNP was used. The enhancing and suppressive effects were not exerted by entirely the same sets of antibodies. The suppression was correlated to efficient antigen binding, but not complement activation, haemagglutination, or isotype of the monoclonal antibodies. In contrast, enhancement was correlated to isotype and complement activation but not to antigen binding capacity. Both the enhancing and the suppressive effects seem to require Fc-mediated functions of the IgG molecules since they modulate the anti-carrier response although they recognize hapten determinants. Thus, one and the same monoclonal hapten-specific IgG-antibody can enhance the anti-KLH response up to 38-fold whereas it suppresses the anti-SRBC response by more that 10-fold.