Mouse hybridomas generated by fusion between a lactate dehydrogenase-B (LDH-B)-specific B10.A(2R) T suppressor (Ts) cell line and the BW5147 thymoma secrete two suppressor factors, TsF-A and TsF-E. The factors carry the same antigen-binding chains but different major histocompatibility complex (MHC) chains (A beta-like and E beta-like, respectively). The TsF-A suppresses the proliferation of A-restricted, LDH-B-specific T helper (Th) cells. In this report we demonstrate that the addition of the TsF-E (isolated on immunosorbent columns with Ek- or Jk-specific antibodies) to the culture of LDH-B-primed B10.A(2R) lymph node cells turns the nonresponder (suppressed) cultures into proliferating ones, and that this change is antigen specific. This enhancing effect occurs in the early phase of the cell culture; the factor has no effect when added 2 days after the initiation of the culture. Because pretreatment of the Ly-2+ but not of the Ly-1+2- cells with TsF-E induces responsiveness, it is very likely that the targets of the factor are the Ts cells or their precursors that belong to the Ly-2+ subset. An incubation period of about 4 h is necessary for the TsF-E to exert its action. The enhancing effect of the TsF-E is abrogated by monoclonal antibodies specific for Ek and Jk antigenic determinants. However, only some of the antibodies that retain the TsF-E on the immunosorbent column neutralize the factor in a functional test. Antibody blocking studies also indicate that the MHC determinants involved in the interaction between the antigen-presenting and the Ts cell during Ts cell activation, and in the TsF-E Ts cell interaction are either very similar or identical. We interpret the data as indicating that the Ts cells or their precursors recognize the TsF-E with the same receptors as they use for the recognition of LDH-B together with the Ek on the antigen-presenting cells. The recognition of the TsF-E inactivates the Ts cell so that proliferation of Th cells then occurs unhindered. Thus, the production of the TsF-E may provide a feedback mechanism that regulates the activation of the Ts cells and, consequently, the degree of suppression in the response to LDH-B.