Considerable effort is being invested in antibody gene technologies for production of species-adapted anti-T cell antibodies which can overcome formation of neutralizing anti-antibodies (anti-Ab). By establishing a mouse model for the generation of syngeneic anti-T cell MoAb, we addressed the question of whether ideally species-adapted T cell-binding antibodies can prime mice to produce anti-Ab. Two anti-Thy-1.2 MoAbs of IgG2a (MmTC) or IgM (MmTC-IgM) isotype were generated in congenic C57B1/6-Thy-1.1 mice. Three injections of MmTC in C57B1/6-Thy-1.2 or (C57B1/6-Thy-1.2xC57B1/6-Thy-1.1)F1 hybrids where they act as syngeneic anti-T cell MoAbs induced low anti-Ab. When MmTC was injected as Igh-mismatched antibody in CBA/J mice, high titre anti-MmTC antibodies were measured and fully mismatched skin allografts rejected within 26 days. MmTC injected twice weekly in syngeneic C57B1/6 mice induced low anti-MmTC and prolonged graft survival up to day 117. In contrast, retreatment induced anti-MmTC with graft rejection within 32 days. Thus, upon retreatment, the immunosuppressive effects of MmTC were inhibited to a similar extent as that seen after antibody treatment in Igh-mismatched mice. However, a recently analysed immunological principle to suppress anti-Ab against T cell binding allo- or xenoantibodies by preinjection of T cell-depleting antibody with species differences in heavy chains also suppressed syngeneic anti-Ab after MmTC retreatment. This effect was accompanied by prolonged graft survival. Thus, our data indicate that inhibitory anti-Ab must be considered in humans even when treated with fully species-adapted anti-T cell MoAb, but may be suppressed by preinjection of a Fc region-mismatched anti-T cell antibody.