Previously, we have demonstrated T-B cell interactions mediated by T cell recognition of immunoglobulin (Ig) peptide/major histocompatibility complex (MHC) class II complexes derived by the B cell processing of endogenously synthesized Ig molecules. In this report Ig-specific T-B cell interaction mediated by B cell presentation of idiotopes (Id) of anti-sIg antibodies to Id-specific T cell clones has been studied in Ig kappa-1-congenic rat strains. A panel of August (RT-1c; Ig kappa-1a) rat T helper clones specific for Id of syngeneic anti-Ig kappa-1b C3B9 monoclonal antibody (mAb) has been developed to study IdC3B9 presentation by Ig kappa-1b-bearing B cells from congenic August.1b (RT-1c; Ig kappa-1b) rats. Five of seven IdC3B9-specific T clones responded even at very low concentrations (100-200 pg/ml) of C3B9 mAb presented by Ig kappa-1b+ B cells. In contrast, the presentation of intact C3B9 mAb by nonspecific antigen-presenting cells (macrophages, Ig kappa-1a+ B cells, etc.) to IdC3B9-specific T cells was of low efficiency. The IdC3B9-specific T cell response to idiotopes of anti-Ig kappa-1b C3B9 mAb was found to be restricted by RT-1B molecule and required the processing of intact C3B9 molecule. IdC3B9 epitope recognized by C31 and C5 clones was mapped to the heavy chain of C3B9 mAb. Thus, B cells are able to present peptides related to the V region of anti-sIg Ab, i.e. Id peptide/MHC class II complexes, to Id-recognizing T cells. IdC3B9-presenting B cells are specifically activated both to proliferation and Ig production upon interaction with IdC3B9-specific T clones. Based on the results of our studies on B cell presentation of Ig epitopes to T cells a hypothetical model of Ig peptide-driven T-B cell interaction has been proposed.