Regulation of the humoral immune response by fragments of complement component C3 has been shown to play an important role in host defense. In this study, we investigated expression of the C3a receptor (C3aR) by human B cells and the effects of C3a and C3a(desArg) on IgG, TNF-alpha, and IL-6 production in Staphylococcus aureus Cowan strain I (SAC)/IL-2-activated B cells. Here we report that tonsil-derived human B cells express a C3aR that is indistinguishable at both the protein (by flow cytometry) and mRNA (by reverse transcription-PCR) levels from that on human neutrophils or on C3aR-transfected mouse L cells. Incubation of SAC/IL-2-activated B cells with C3a or C3a(desArg) resulted in a dose-dependent suppression of the polyclonal immune response. Under these same experimental conditions, IL-6 and TNF-alpha release was suppressed in a dose-dependent manner. Kinetic studies with SAC/IL-2-activated B cells revealed that C3a must be present at initiation of the culture to exert its suppressive effect on IgG and IL-6 production. These results demonstrate that tonsil-derived B cells express the C3aR and that C3a as well as C3a(desArg) have a direct immunomodulatory effect on these cells.