Feedback regulation of the primary humoral immune response to sheep erythrocytes (SRBC) was studied in vitro. Whole spleen cells or spleen cell subpopulations were incubated with antigen for 4 d under Mishell-Dutton conditions (education) and the surviving cells tested for regulatory activity in fresh anti-SRBC spleen cell cultures assayed by measuring plaque-forming cells on day 4. The data indicate that (a) whole spleen cells educated with SRBC exert potent antigen-specific suppression in the assay culture, (b) surface Ig- (sIg-) cells (T cells) prepared by either nylon-wool separation or fractionation on rabbit anti-mouse-Ig-coated polystyrene Petri dishes failed to generate suppressive activity when educated alone, in 2-mercaptoethanol, or in the presence of additional macrophages, (c) surface Ig (sIg+) (B) cells educated alone also failed to generate suppressor cells, and (d) mixing sIg- (T) and sIg+, Lyt 123- (B) cells reconstituted the ability to induce suppressor cells under these conditions. The antigen-primed cell actually required to transfer suppression was also characterized by separating cells using anti-Ig coated dishes, by fluorescence-activated cell sorting and by anti-Lyt treatment. All these methods clearly identified sIg+ (B) and not sIg+ (T) cells as the important educated cells. It is concluded that under our conditions, T cell-dependent B cells triggered by antigen during primary in vitro cultures cause potent specific feedback suppression of humoral responses. Possible mechanisms for this suppression, including antigen blockade or anti-idiotypic responses, are discussed.