Co-stimulation of B lymphocytes with IL-4 plus nonmitogenic concentrations of anti-Ig antibodies, or protein kinase C (PKC) activators, drives resting B cells into DNA synthesis. Although cross-linking of the sIg receptors provokes the generation of the intracellular second messengers, inositol 1,4,5-trisphosphate (IP3) and diacylglycerol, the molecular mechanism utilized by IL-4R in murine B cells has not, as yet, been defined. In human B cells IL-4 has been shown to induce a transient rise in IP3 followed by a sustained elevation of cAMP. However, in murine B cells, IL-4 does not induce the release of IP3, Ca2+ mobilization, PKC translocation, or indeed modify signaling via the phosphoinositide pathway induced by ligation of sIg receptors. We now present evidence that, in murine B cells, IL-4 synergizes with nonmitogenic concentrations of anti-Ig to provoke translocation of PKC from the cytosol to membranes. In addition, the lymphokine up-regulates PKC levels and activity and prevents phorbol ester-induced PKC down-regulation in B cells. We therefore propose that (unknown) signals generated via IL-4R potentiate and/or prolong sIg-induced PKC activation. These observations may therefore provide a biochemical basis for explaining how IL-4 and anti-Ig synergize to induce B cell activation.