Background: Signaling through CD40 activates multiple kinases and signal pathways that drive diverse CD40-mediated biologic functions. The specific pathways activated by CD40 signaling involving CD40-dependent Ig class switch recombination (CSR) have not been defined. Objective: We sought to dissect CD40-activated signaling required for CD40-mediated Ig CSR by using the specific signal pathway inhibitors, with the emphasis on CD40-activated p38 mitogen-activated protein kinase (p38 MAPK) signaling in CD40-mediated CSR to IgE. Methods: Human B cells were costimulated with IL-4 plus anti-CD40 in the presence or absence of specific signal pathway inhibitors. Ig production, kinase phosphorylation, IgH ϵ germline transcripts and Sμ-Sϵ recombination were examined, and their relationships were analyzed. Results: CD40-dependent IgE induction was inhibited by the specific p38 MAPK inhibitor SB203580 but not by the extracellular signal-regulated protein kinase-specific inhibitor PD98059 or the phosphatidylinositol 3-kinase-specific inhibitor LY294002. CD40 activation of p38 MAPK correlated with CD40-dependent IgE production, and IgE suppression by SB203580 correlated with the inhibition of CD40-activated p38 MAPK phosphorylation. Suppression of IgE production by SB203580 was not due to inhibition of cell proliferation because SB203580 did not suppress IL-4 plus ⍺-CD40-induced cell proliferation. SB203580, but neither PD98059 nor LY294002, inhibited CD40-dependent Sμ-Sϵ recombination, as determined by using a digestion circularization PCR assay. The inhibitory effects of SB203580 on IgE production and Sμ-Sϵ recombination were directly related to its ability to suppress production of Ig ϵ germline transcripts. Conclusion: These results demonstrate that p38 MAPK is required for CD40-mediated class switching to IgE. (J Allergy Clin Immunol 2002;110:421-8.)