Following induction of reversed passive Arthus reactions in rats, extracts of tissue lesions were assessed for the presence of leukotactic activity for neutrophilic granulocytes. During the first 3 hr of the Arthus reaction, extracts were rich in leukotactic (neutrophil) activity which, on the basis of antigenic and ultracentrifugal analysis, could be ascribed to the C5-related leukotactic factors and C5 cleavage products. The injection of radiolabeled human C5 into rats resulted in the incorporation of the radiotag into the leukotactic activity as assessed by fractionation of extracts in sucrose density gradients. No such finding occurred when human C3 was injected into rats under similar conditions. As shown by complement-depleting procedures, development of leukotactic activity in Arthus reactions was complement-dependent. When rats were depleted of circulating neutrophils, leukotactic activity was found in extracts of reaction sites, albeit at considerably reduced levels. A C5-dependent generator of leukotactic activity was found in lysosomal granules of rat neutrophils and may account for a portion of the leukotactic activity generated in Arthus reactions. These studies emphasize the phlogistic role of complement products in reactions of immunologic vasculitis.