Abstract. Using a combined in vivo and in vitro approach, we have analyzed the immunofluorescent localization and function of a 140,000-mol-wt glycoprotein complex implicated in cell adhesion to fibronectin (FN), with particular emphasis on neural crest cell adhesion and migration. This putative fibronectin receptor complex (FN-receptor) was detectable in almost all tissues derived from each of the three primary germ layers. It was present in both mesenchymal and epithelial cells, and was particularly enriched at sites close to concentrations of FN, e.g., at the basal surfaces of epithelial cells. It was also present on neural crest cells.The distribution and function of this putative receptor was then analyzed on individual cells in vitro. It was diffusely organized on highly locomotory neural crest cells and somitic fibroblasts. Both motile cell types also displayed relatively low numbers of focal contacts and microfilament bundles and limited amounts of localized vinculin, a-actinin, and endogenous FN. In contrast, the FN-receptor in stationary embryonic cells, i.e., somitic cells after long-term culture or ectodermal cells, existed in characteristic linear patterns generally co-distributed with a-actinin and fibers of endogenous FN. Anti-FN-receptor antibodies inhibited the adhesion to FN of motile embryonic cells, but not of stationary fibroblasts. However, these same antibodies adsorbed to substrata readily mediated adhesion and spreading of cells, but were much less effective for cell migration.Our results demonstrate a widespread occurrence in vivo of the putative FN-receptor, with high concentrations near FN. Embryonic cell migration was associated with a diffuse organization of this putative receptor on the cell surface in presumably labile adhesions, whereas stationary cells were anchored to the substratum at specific sites linked to the cytoskeleton near local concentrations of FN-receptor. D URING embryonic development, interactions of cellswith extracellular matrix are important in regulating cell behavior (27,59,65). The extracellular molecule fibronectin (FN) ~ promotes the adhesion, spreading, and formation of specialized adhesion sites in a variety of cells (25,31,40,49). Besides this role, FN stimulates the in vitro locomotion of several embryonic cell types, including chick heart fibroblasts and avian neural crest cells (17,42,43,48). In vivo, the presence of FN has often been correlated with the migration of cells, e.g., gastrulating cells, primordial germ cells, and neural crest cells (5,18,21,22,28, 39,50,58,64). In addition, the interaction of FN with the cell surface is a prerequisite for cell movement, since blocking the FN cellAbbreviations used in this paper." DME, Dulbecco's modified Eagle's medium; FN, fibronectin; FN-receptor, putative fibronectin receptor complex; NCS, newborn calf serum.binding domain inhibits the migration of cells (6,7,48). These observations imply that the same molecule is involved in both the transient adhesions involved in cell movement and in the firm ancho...