The relative role of complement and CD14 in Escherichia coli-induced leukocyte CD11b up-regulation, phagocytosis, and oxidative burst in human whole blood was examined. The highly specific thrombin inhibitor lepirudin was used as anticoagulant, as it does not affect complement activation. Complement inhibition at the level of C3 (anti-C2 and anti-factor D) and C5 (C5a receptor antagonist and anti-C5/C5a) efficiently inhibited CD11b up-regulation, phagocytosis, and oxidative burst in granulocytes. Monocyte activation was generally less complement-dependent, but when C3 activation was blocked, a pronounced inhibition of phagocytosis and oxidative burst was obtained. Only the combination of anti-C2 and antifactor D blocked E. coli C3 opsonization completely. Whole E. coli, disrupted E. coli, and the C3-convertase activator cobra venom factor up-regulated CD11b rapidly on both cell types, proportional to their complement activation potential in the fluid phase. In comparison, purified LPS at concentrations comparable with that present in the E. coli preparations did not activate complement. Oxidative burst was induced only by whole bacteria. Finally, the combination of complement inhibition and anti-CD14 completely blocked E. coli-induced granulocyte and monocyte CD11b up-regulation and quantitatively, virtually abolished phagocytosis. The results indicate that complement and CD14, despite differential effects on granulocytes and monocytes, are the two crucial, quantitative factors responsible for E. coli-induced CD11b, phagocytosis, and oxidative burst in both cell types.