The reduction of nitroblue tetrazolium dye by human neutrophils was measured in the presence of serum in which the complement system had been activated through the alternate pathway by interaction with inulin. Neutrophils incubated with serum inulin supernatants reduced the dye and showed a general increase in oxidative metabolism. The oxidation of glucose-1-14-C by supernatant prepared from selectively depleted sera indicated that the neutrophil-stimulating factor(s) was generated through the alternate pathway of complement activation. The possibility that inulun had been ingested as a particle was ruled out by light microscopy and radiolabeling studies. The failure of neutrophils stimulated by the serum-inulun supernatants to migrate after exposure to a chemotactic agent suggested that the site of neutrophil-complement interaction was on the cell membrane. It is concluded from these results that biologically active fragments generated through the alternative pathway of complement activation can stimulate neutrophil metabolism in the absence of phagocytosis. Interaction of such fragments with circulating neutrophils in vivo and the subsequent metabolic activation of these cells is one explanation for the spontaneous reduction of nitroblue tetrazolium dye in vitro by neutrophils from patients with certain infections and inflammatory disorders.