Cell surface assembly of the membrane attack complex (MAC) of complement occurs in a variety of pathophysiological settings. Depending upon the density and size distribution of pores formed by the MAC and the functional integrity of membrane regulators of complement activation, the MAC can either cause direct cell lysis or transduce cell activation. We have examined the functional capacity of sublytic concentrations of MAC to induce the secretion of specific alpha- and beta-chemokines from human umbilical vein endothelial cells (HUVECs). Endothelial cell activation by the MAC has particular relevance to complement-dependent inflammatory processes including ischemia-reperfusion injury and acute lung injury. Assembly of sublytic concentrations of the MAC on HUVECs resulted in the sequential secretion of both neutrophil and monocyte chemotactic activities. Analysis of conditioned medium from MAC-bearing HUVECs revealed that the neutrophil chemotactic activity was largely attributable to interleukin (IL)-8, whereas the monocyte chemotactic activity, which was detected later (peak at 8 hours versus 4 hours), was largely attributable to MCP-1. This temporal pattern of MAC-induced secretion of IL-8 and MCP-1 was confirmed using IL-8- and MCP-1-specific enzyme-linked immunosorbent assays. Northern hybridization analysis of HUVECs revealed that MAC deposition was accompanied by an increase in IL-8 and MCP-1 mRNA levels. These data indicate that assembly of sublytic concentrations of the MAC on HUVECs can induce the sequential secretion of both neutrophil and monocyte chemotactic activities and that the former is largely attributable to IL-8 whereas the latter is largely attributable to MCP-1.