Soluble anti-Ig or anti-mu antibodies completely abrogate the lipopolysaccharide (LPS)-stimulated proliferation of purified B cells obtained from spleens of 5-7 day old mice. This provides further evidence for the powerful nature of the negative signals transduced by sIgM receptors on immature B cells. In addition, ligation of sIgD receptors (expressed by approximately 30% of these cells) by two out of three monoclonal anti-delta antibodies inhibits the response to LPS by some 50%. Ligation of sIgD on purified sIgD+ B cells (greater than 98% sIgD+) completely inhibited the LPS response of these cells. The inclusion of IL-4 in these cultures partially (with anti-mu), or completely (with anti-delta), restored the proliferative response. Immobilized anti-mu or anti-delta caused comparable levels of inhibition as the soluble antibodies: IL-4 again rescued the inhibition caused by immobilized anti-delta, but not that induced by anti-mu. These results therefore indicate that engaging either sIgM or sIgD receptors on developing B cells delivers negative (tolerogenic) signals. They also implicate IL-4 as a major (although not the only) T cell-derived influence which can modulate these signals. Finally, the data suggest that extensive cross-linking of sIgM (by immobilized anti-mu) causes more profound unresponsiveness in immature B cells, which cannot be rescued by IL-4 alone.