Timing, magnitude, and effect of the murine antibody response to rat pan-T-cell antibodies were studied in a bone marrow (C57BL/6-to-CBA mice) transplantation model. Prospective C57BL/6 marrow donor mice were sensitized against pan-T-cell (Thy-1, Thy-1.2, Lyt-1) monoclonal antibodies of various rat isotypes or against polyclonal rat antimouse-thymocytes (rat ATG) antibodies. Three days prior to transfer of spleen and bone marrow cells, the sensitized C57BL/6 donors received a dose of anti-Thy-1 mAb (RmT1) known to abolish graft-versus-host reactivity of unsensitized donors. The injected mAb provoked anti-antibodies reacting with RmT1. The anti-antibodies inhibited immunosuppression of the rat mAb RmT1 even if they bound only to nonidiotypic epitopes on RmT1. Avoiding cell-binding of the sensitizing rat anti-Thy-1.2 mAb by its injection into Thy-1.1 mice induced only low-titer and delayed anti-antibodies. This indicated the enhanced immunization potential of anti-Thy-1 when bound to cells. Finally, sensitization leading to the mouse antirat anti-antibodies and reversion of immunosuppression was prevented or reduced considerably by T cell depletion with a mouse IgG2a anti-Thy-1.2 mAb or high-dose cyclophosphamide or by rabbit ATG, provided it was initiated before starting the sensitizing injections of the rat antimouse T cell antibodies.