It was reported that dialysate (BWD) separated from the aqueous extract of buckwheat was a haptenic substance capable of neutralizing IgE antibody on mast cells and that the activity was specific to non-dialysate (BWND)-induced hypersensitivity reaction. The effect of BWD on antibody formation was investigated in the present paper. In rats, anti-buckwheat IgE formation was slightly depressed by the administration of BWD, but both anti-DNP IgE and IgG formations in rats immunized with DNP-BWND were unaffected. In mice, anti-buckwheat IgE formation was suppressed by BWD administration. A good correlation was noted between a decrease of surface IgE population on B cells and that of IgE titer in serum. However, the helper function of T cells for adoptive anti-DNP IgE formation was little affected by BWD treatment. Lymphocyte transformation to BWND and other non-specific mitogens using spleen cells obtained from sensitized mice was inhibited in a dose-dependent fashion by the addition of BWD. However, lymphocyte transformation using spleen cells pretreated with BWD was not affected by other non-specific mitogens except for BWND and pokeweed mitogen.