Reactive oxygen species (ROS) produced by the innate immune system work as effectors to destroy pathogens and to control cellular responses. However, their role in the adaptive immune response remains unclear. Here we studied the effect of exogenous ROS on CD40-induced B cell activation. H2O2 treatment inhibited CD40-induced immunoglobulin production of B cells, DNA binding of NF-kappaB, IkappaBalpha degradation and IKK phosphorylation. On the other hand, H2O2 treatment did not induce obvious B cell death after 30 min of stimulation. Although the ligation of anti-CD40 antibody was not disturbed by H2O2, TRAF2 recruitment to CD40 was inhibited. These results suggest that exogenous ROS play a negative role in CD40 signaling during B cell activation.