"Fibronectin" is a term describing a class of immuno-chemically related glycoproteins that are found in blood, in connective tissues, and in most basement membranes. All types of fibronectins have distinct binding domains that serve to mediate an adhesive function between collagen, cells (e.g., fibroblasts, macrophages), fibrin(ogen), and/or certain glycosaminoglycans (e.g., heparin). Monocytes and macrophages are phagocytic cells which collectively comprise the "mononuclear phagocytic" system. Plasma fibronectin (CIg) mediates: a) the attachment of monocytes to gelatin-coated surfaces; and b) the attachment of gelatin-coated RBCs or latex particles to surface-bound monocytes. This process was mediated only by surface-bound forms of fibronectin. Particle attachment to cells, per se, was not associated with augmented particle ingestion, although particle binding did result in increased expression of the monocyte C3 and Fc receptors. These data indicate that monocytes have surface receptors for fibronectin. It appears that the relatively strong affinity between fibrin and fibronectin may provide a mechanism for recognition and subsequent clearance of fibrin-fibrinogen complexes from the blood by attachment to fibronectin receptors on mononuclear phagocytes. One can also speculate that fibronectin binds to exposed or denatured collagen at sites of injury, leading to macrophage attachment and differentiation. Such events probably play an important role in wound organization and healing.