To study mechanisms and mediators regulating the distribution of intravascular granulocytes between circulating and marginated pools, a human model with extreme transient margination, the neutropenia of continuous flow filtration leukophoresis, was analyzed. Studies in animals demonstrated the existence of a complement (C)-derived granulocytopenia-inducing factor. Thus, autologous plasma, exposed to nylon fibers (NF) of the filtration system, produced an acute selective decrement of circulating granulocytes and monocytes. This phenomenon was blocked by decomplementing plasma, by pretreatment of plasma with EDTA or hydrazine, and by preheating at 56°C, but did occur after recombination of heat-inactivated and hydrazine-treated plasma before NF exposure. Preheating plasma at 50°C did not inhibit the neutropenic response, suggesting involvement of the classical pathway of C activation. Ultrafiltration studies indicated that the NF-provoked neutropenia-inducing factor has a mol wt in the range of 10,000-30,000, and is heat stable (56°C). To analyze the hypothesis that C-induced neutrophil margination might be consequent to increased cell adhesiveness to endothelial surfaces, the role of C in promoting granulocyte adherence was evaluated in vitro. Measured with a plastic Petri dish assay, granulocyte adherence was significantly reduced in heat-inactivated (56°C) and hydrazine-treated plasma, but adherence promoting capacity was restored by mixing the two plasmas, or by adding purified C3 to hydrazinetreated plasma. After exposure to activated C, neutrophils showed significantly increased adhesiveness which was maintained when cells were resuspended in heat-inactivated plasma, but progressively lost when resuspended in fresh plasma. On the basis of these results we conclude that granulocyte adhesiveness in vitro and margination in vivo are closely associated, C-dependent phenomena.