Human alveolar macrophages (AMs) migrated toward and aggregated to C5a but not toward C5a-deficient serum. Human AMs from cigarette smokers migrated significantly farther than did AMs from nonsmokers. Human AMs phagocytized Escherichia coli opsonized with activated C3. Immunoglobulin G was not required for phagocytosis. Human AMs demonstrate chemotaxis and aggregation to C5a. Further, human AMs can phagocytize bacteria coated with activated C3.