It is well-established that preactivated CD4+ T cells can activate mature B cells in a polyclonal, MHC-unrestricted fashion. We have used this system to investigate the effects of T cell-derived signals on immature B cells purified from the spleens of neonatal mice, since these cells are unresponsive to many polyclonal activators and are exquisitely sensitive to tolerization. We show that immature B cells can be induced to proliferate by anti-CD3 activated, fixed Th1 and Th2 cells, although the latter induce a greater response than the former. Antibodies to IL-4 partially blocked stimulation by Th2 cells, whereas antibodies to IL-2 and IL-5 had no effect on responses to Th1 cells. This suggested that molecules in addition to IL-4 contribute to the capacity of T cells to induce B cell activation, one likely candidate being the ligand for CD40. We therefore generated mouse erythroleukemia (MEL) transfectants which express CD40 ligand (CD40L). These transfectants also induced proliferation of immature B cells, which is enhanced by IL-4. Unlike the situation with mature B cells, both anti-mu and anti-delta antibodies inhibited the activation of immature B cells by CD40L-MEL cells. However, this inhibition was reversed by IL-4, which synergized with signals delivered through CD40 to render immature B cells refractory to negative signals delivered through sIg. Taken together these data suggest that immature B cells can be activated by T cell-derived contact signals and that CD40L-CD40 interactions, in the presence of IL-4, are capable of abrogating the negative signals generated via sIgM and sIgD receptors expressed by these cells.