Tolerance to the minor lymphocyte stimulating (Mls) self-antigens has been shown to be due to the intrathymic deletion of T-cell clones bearing certain T-cell receptor (TcR) V beta regions. T cells bearing these V beta regions (V beta 6, V beta 7, V beta 8.1, V beta 9) are deleted in Mlsa-positive mice. This report demonstrates that hyperimmunization of AKR mice with anti-V beta 6 breaks tolerance to the self-antigen Mlsa and induces the development of anti-Mlsa antibodies. These antibodies had the capacity to block Mlsa-induced mixed lymphocyte responses between H-2 identical strain combinations and also between Mlsa-congeneic BALB.D2MA and parental BALB/c strains. Furthermore, when the responder lymphocytes were Mlsa-positive their response to allogeneic major or minor histocompatibility antigens was enhanced in the presence of this antiserum. This indicates that Mlsa may be present on T cells and that occupancy of this ligand leads to enhanced signal transduction.