In leukocytes, C3a and C5a cause chemotaxis in a G(i)-dependent, pertussis toxin (PT)-sensitive fashion. Because we found that HUVECs and immortalized human dermal microvascular endothelial cells express small numbers of C3aRs and C5aRs, we asked what the function of these receptors was on these cells. Activation of the C3aR caused transient formation of actin stress fibers, which was not PT-sensitive, but depended on rho activation implying coupling to G(alpha12) or G(alpha13). Activation of the C5aR caused a delayed and sustained cytoskeletal response, which was blocked by PT, and resulted in cell retraction, increased paracellular permeability, and facilitated eosinophil transmigration. C5a, but not C3a, was chemotactic for human immortalized dermal microvascular endothelial cells. The response to C5a was blocked by inhibitors of phosphatidylinositol-3-kinase, src kinase, and of the epidermal growth factor (EGF) receptor (EGFR) as well as by neutralizing Abs against the EGFR and heparin-binding EGF-like factor. Furthermore, immune precipitations showed that the EGFR was phosphorylated following stimulation with C5a. The C5aR in endothelial cells thus uses a signaling cascade-transactivation of the EGFR-that does not exist in leukocytes, while the C3aR couples to a different G protein, presumably G(alpha12/13).