The humoral immune response against α(1 → 3) dextran (Dex) in BALB/c mice is characterized by the formation of predominantly IgM antibodies bearing the J558 idiotype. IgG antibodies do not appear in euthymic mice. In athymic animals however, the response proceeds to a vigorous IgG production. In euthymic mice formation of IgG is suppressed by J558 idiotype‐ specific regulatory T cells recognizing in association with I‐Ed and in cognate T/B interaction the VH CDR3 derived peptide of the J558 idiotpye. Only B‐2 lymphocytes produce IgG whereas B‐1 cells do not participate in the production of this Ig class. Using a novel synthetic all α(1 → 3)‐D‐gluco configurated tetrasaccharide the Dex‐specific B cells can for the first time be analyzed in FACS. In experiments using this newly designed low molecular Dex no signs of B cell apoptosis can be found. This demonstrates a true silencing of persisting Bγ memory cells and supports previous by adoptive transfer experiments. In this suppression an involvement of CD28/B7–1 interaction can be demonstrated which is a necessary costimulatory suppression signal in addition to the cognate TCR/peptide‐I‐Ed interaction between J558 Id‐specific T cells and J558 idiotype beating B cells. This results in an activation of 178–4 Ts cells, leading to an overall suppression of the Dex‐specific IgG isotype production on the one hand and on the other hand provides a signal for the survival and clonal expansion of J558 Id‐positive B cells.