The major cell surface glycoprotein of chick embryo fibroblasts, cellular fibronectin (formerly known as CSP or LETS protein), was purified and used to produce monospecific antisera. After affinity purification, the anti-fibronectin was used to investigate fibronectin's localization, its transfer from intracellular to extracellular pools, its antibody-induced redistribution on the cell surface, and its role in cell shape. Anti-fibronectin localizes to extracellular fibrils located under and between sparse cells, and to a dense matrix that surrounds confluent cells. Cellular fibronectin is also present in granular intracytoplasmic structures containing newly synthesized fibronectin before secretion. This intracellular staining disappears 2 h after treatment with cycloheximide or puromycin, and returns after removal of these protein synthesis inhibitors. In pulse-chase experiments using cycloheximide, fibronectin was sequentially transferred from the intracellular to the fibrillar extracellular forms. Transformation of chick fibroblasts results in decreases in both extracellular and intracellular fibronectin, and in altered cell shape. Treatment of untransformed chick fibroblasts with anti-fibronectin results in rapid (30 min) alteration to a rounder cell shape resembling that of many transformed cells. These rapid shape changes are followed by a slow, antibody-induced redistribution of fibronectin to supranuclear caplike structures. This "capping" is inhibited by metabolic inhibitors. Reconstitution of cell surface fibronectin onto transformed cells restores a more normal fibroblastic phenotype. The reconstituted fibronectin on these cells organizes into fibrillar patterns similar to those of untransformed cells. As with untransformed cells, treatment of these reconstituted cells with anti-fibronectin also results in cell rounding and "capping" of fibronectin.