GK1.5, a rat anti-mouse CD4 mAb, is effective in the treatment of several autoimmune syndromes, induces tolerance to co-administered Ag, and prolongs allograft survival. We have constructed a family of molecules with GK1.5 V regions and mouse gamma 1, gamma 2a, gamma 2b, or gamma 3 constant regions to investigate the mechanisms underlying the effectiveness of GK1.5. The rat-mouse chimeric antibodies are specific for murine CD4 and have identical binding curves as rat GK1.5 on CD4+ T cells. The chimeric GK1.5 gamma 2a, GK1.5 gamma 2b, and GK1.5 gamma 3 antibodies are more efficient than rat GK1.5 at C-mediated cytotoxicity. This is attributed to the enhanced capacity of the chimeric antibodies, compared to rat GK1.5, to lyse CD4+ cells with a low cell surface Ag density. This observation may have important implications for therapy.