The effect of complement (C) components on the intracellular killing of E. coli 0111:B4 by human PMN was studied. Various intermediate bacteria were prepared by opsonizing IgM-coated 0111:B4 with yeast cell-treated human serum (BAC1-3), a C6-deficient human serum (BAC1-5), a C8-deficient human serum (BAC1-7), a C8-deficient human serum, and with partially purified C8 (BAC1-8). All these bacterial preparations were phagocytosed by human PMN, but only BAC1-8 and, to a lesser extent, BAC1-5 were killed. Similar results were obtained when the 400 x G postnuclear supernatant (PNS) of PMN homogenate was used instead of intact leukocytes. The C9 nature of the killing factor in the PMN homogenate was ruled out by its inability to lyse EAC1-8 and by the finding that the killing of BAC1-8 by the PMN factor was not inhibited by the antiserum against human C9. The anti-C5 and anti-C8 antisera were unable to inhibit the killing by the PNS of BAC1-5 and BAC1-8, respectively, suggesting that bound C5 and C8 do not provide a binding site for the killing factor.