To investigate the modulatory function of complement component C5a in macrophages, the effects of C5a on macrophage phagocytosis were examined by means of fluorescent polystyrene particles and flow cytometry. We found that C5a enhanced the phagocytosis of latex beads in mouse peritoneal macrophages. The results suggest that the anaphylatoxin C5a can modulate macrophage functions and may play an important role in specific immune responses as well as in a non-specific immune mechanism.