Ag-specific B cells from the peripheral blood of immunized donors were separated following rosette formation with tetanus toxin (TT)-coupled immunobeads to study the regulation of T cell-dependent Ab production. Surface IgM+ (sIgM+) B cells rosetting with TT-coated beads produced anti-TT IgM, IgG, and IgE Abs in cultures stimulated with EBV or anti-CD40 Ab in the presence of IL-4; this was associated with increased numbers of B cells positive for sIgG and sIgE. TT-binding sIgM+ B cells also produced significant amounts of anti-TT IgM, IgG, and IgE Abs when they were cultured with autologous T cells in the presence of IL-4. Addition of TT to the IL-4-stimulated, T cell-dependent cultures resulted in higher levels of TT-specific IgM and IgG Abs than observed with T cells alone, but the inclusion of TT with T cells suppressed anti-TT IgE production. Addition of TT Ag to cultures with allogeneic T cells failed to enhance IgM and IgG responses, and the inhibition of IgE production was reduced compared with cultures containing autologous T cells in the presence of Ag. These results demonstrate that the presence of Ag contributes to the regulation of Ab production, resulting in enhanced IgM and IgG responses, but down-regulation of IgE Ab production.