Helper T (Th) cell-dependent activation requirements for 2,4,6-trinitrophenyl (TNP)-specific resting B cells obtained from mice transgenic for Sp-6 mu, kappa genes were analyzed. Carrier-specific T cell help required linked recognition of TNP carrier and was functionally restricted by the B cell major histocompatibility complex. However, histoincompatible T cell-B cell conjugates formed by bridging surface immunoglobulin and Th cell receptor for antigen (TcR) through TNP-conjugated anti-TcR antibodies resulted in the efficient differentiation of TNP-specific B cells. Thus, Th cell-dependent cognate recognition of B cells is not obligatory. Specific conjugate formation could be obviated by using unconjugated fragments of anti-TcR antibodies. If dimeric, these fragments supported the Th cell-dependent differentiation of co-cultured histoincompatible resting B cells. Unconjugated monomeric fragments were ineffective, demonstrating the necessity for TcR cross-linking. Resting B cells from Sp-6+ mice rendered TNP-conjugated monomeric fragments of anti-TcR antibodies effectively multivalent, thereby satisfying conditions for the activation of co-cultured Th cells. The results demonstrate that Th cells do not transduce activation signals through TcR recognition of B cell membrane-associated ligand which limit the induction of B cell differentiation. Cross-linking of TcR on Th cells is required, sufficient and can be induced through interaction with the antigen-specific B cell surface.