Macrophages secrete fibronectin (Fn)' (1-3), bind Fn (3-5), and have been reported to migrate in response to Fn fragments (6). In addition, the interaction of human macrophages with Fn provides a second signal that leads to enhanced complement and Fc receptor-mediated phagocytosis by these cells (7)(8)(9)(10)(11)(12)(13)(14) . The structure ofthe Fn receptor, or receptors, responsible for both adherence to Fn and Fn-mediated phagocytosis enhancement in macrophages is not completely understood . Previous studies have demonstrated membrane proteins on monocytes or culture-derived macrophages that interact with the 110-kD cell-binding domain of Fn (15). These proteins exhibit electrophoretic mobility typical of the recently described Fn receptor, termed VLA-5, purified from placenta, fibroblasts, the MG63 osteosarcoma cell line, erythroblasts, and platelets (reviewed in references 16-18) . This receptor is a heterodimer consisting of an ti 150-kD a chain and a 130-kD a chain under nonreducing conditions (19). Identification and sequence analysis of cDNA clones for both chains from a placental cDNA library has placed this receptor in the integrin family (18,20), whose members function in cellular adhesion . The VLA antigens form a part of this family and are defined by having a common R chain that can noncovalently combine with at least six distinct a chains (21)(22)(23)(24). We have recently demonstrated by Northern blot analysis the presence of mRNA that hybridizes with probes for both the VLA-5 a and the common VLA (3 chain in monocytes, as well as in the monocyte-like cell lines HL-60, THP, and U937 (25). In addition, partial cDNA clones that we have isolated from an HL-60 library have shown identical restriction