An in vitro model for complement (C)-mediated monocyte binding to injured cells is described. This model is based on the property of cytoskeletal intermediate filaments (IMF) to directly and independently activate serum C via the classic pathway. Fixed monolayers of cultured fibroblasts, endothelial cells, and L 132 epithelial cells were each used as targets. The authors first subjected the target cells to limited detergent extraction in order to make permeable the plasma membrane, and then exposed them to normal human serum. Immunohistologic analysis demonstrated that this process permitted binding of clearly demonstrable amounts of C1q, C4, and C3 to cytoplasmic loci containing IMF. The target cells were then incubated for various lengths of time with peripheral blood mononuclear cells containing about 20% monocytes. Preferential binding of monocytes was demonstrated by histochemical and immunologic staining methods. Only 5% of the attached cells were B lymphocytes. By transmission electron microscopy the attached cells exhibited typical monocyte morphology. Attachment to target cells involved contact with areas rich in IMFs and led to signs of activation; both an increased number of cytoplasmic organelles and phagocytosis of target cell material were observed. Attachment of monocytes was clearly dependent on serum treatment of the targets; the ratio was 10:1 when compared with control experiments using target cells that had not been incubated with serum. It is reasonable to consider that bound monocyte-derived macrophages associate with activated C components, most likely C3b fragments or C1q, for which monocyte surface receptors have been established. This mechanism may be triggered in various inflammatory reactions involving cell and tissue injury and as an event in the final stages of macrophage-mediated removal of injured or nonviable cells.