One component of the B cell activation cascade is the induction of the protooncogene c-fos. Data presented here demonstrate that stimulation of mIgM-bearing cells with either anti-IgM or the combination of PDB plus ionomycin generated comparable levels of c-fos RNA. Furthermore, a synergistic response was observed when the cells were treated with selected concentrations of anti-IgM plus either PDB or ionomycin. In contrast, stimulation of mIgG-bearing B cells with anti-IgG did not induce the c-fos RNA levels that were seen when these cells were treated with the combination of PDB plus ionomycin. Treatment of mIgG-bearing cells with only the combination of anti-IgG plus ionomycin yielded a synergistic response and anti-IgG plus PDB did not. Thus, induction of c-fos RNA appears to be different in mIgM- and mIgG-bearing B cells after stimulation through mIg.