In contrast to C5a, which represents a well-established potent activator of the respiratory burst in polymorphonuclear neutrophilic granulocytes (PMN), the functional role of C3a in the activation of PMN is, so far, poorly understood. Herein, the potential role of human C3a in the activation of the respiratory burst in human PMN was investigated. The release of reactive oxygen species (ROS) of PMN from healthy donors was measured by lucigenin-dependent chemiluminescence. C3a dose-dependently induced the production of ROS in human PMN in the range between 10 ng/mL and 1,000 ng/mL, whereas C3a-desArg was inactive. Flow cytometric measurement of H2O2 by dihydrorhodamine-123 labeling of anti-CD16-stained PMN showed that predominantly neutrophilic PMN are responsible for the C3a-induced activation of the respiratory burst. To exclude that C3a stimulation was caused by contamination with C5a, the specificity of C3a-induced activation of PMN was shown using monoclonal antibodies (MoAbs). Accordingly, the effect of C3a was completely abolished in the presence of Fab fragments of a blocking anti-C3a MoAb. In addition, blockade of the C5a receptor by the anti-C5a receptor (anti-C5aR) MoAb, S5/1, totally inhibited the C5a-induced production of ROS, whereas the C3a response in the presence of the anti-C5aR MoAb was unaffected. The specificity of the response was further confirmed by homologous desensitization after restimulation with C3a. In contrast, no cross-desensitization was observed upon stimulation with C5a. The C3a-induced ROS production by PMN was inhibited by pertussis toxin, indicating the involvement of guanine nucleotide-binding proteins (Gi proteins) in the signal transduction process initiated by C3a. In addition, stimulation of PMN by C3a resulted in a transient increase in the cytosolic free calcium concentration ([Ca2+]i) in a dose-dependent manner. In contrast to C3a- induced ROS production, C3a did not induce a chemotactic response in PMN, indicating functional qualitative differences as compared with C5a. In summary, these results show that C3a is a potent activator of the respiratory burst in human PMN. Therefore, these findings point to a novel role of C3a in the pathogenesis of inflammatory diseases associated with increased C3a levels and PMN activation.