In the present study we investigated whether autoreactive T cells derived from murine Peyer's patches (PP) have the capacity to regulate mucosal B cell differentiation to IgA-producing plasma cells in vitro. We also examined whether B cell development is mediated by lymphokines from immunoregulatory T cells - that is, B cell stimulatory factors (BSF) and cofactors (coBSF) - which include B cell growth factor (BCGF), putative alpha B cell immunoglobulin (Ig) heavy chain switch factor (BSWF alpha), and B cell differentiation factor (BCDF), as well as interleukin-2 (IL-2). To this purpose we developed in vitro a variety of BSF (especially putative BSWF alpha)-producing autoreactive (self-class II molecules responsive) T cell hybridoma cell clones from murine PP, and studied the functional activity of the BSF, especially a putative alpha Ig heavy chain switch (mu----alpha) factors(s). These T hybridome cell lines possessed the surface phenotypes of Thy 1.2, CD4+, CD5+ and CD8- and produced a variety of BSF, including two kinds of BCGF (IL-5, and a BCGF that did not require additional costimulators to induce proliferation of preactivated B cells), putative BSWF alpha/gamma, BCDF, and/or IL-2. The results strongly support the view that the autologous T cell plays an important role not only in B cell proliferation and terminal maturation, but also in alpha heavy chain switching in PP. This T-B cell interation is mediated at least in part through BSF lymphokines elaborated by the autoreactive T cell, probably activated in situ in the lymphoid tissue microenvironment.