Fibronectin is a major glycoprotein component of normal fibroblasts in culture. External fibronectin is predominantly present in a pericellular fibrillar matrix that mediates distant cell-cell and cell-substratum contacts. A small proportion of external fibronectin is closely associated with the plasma membrane. In the matrix, fibronectin is partially disulfide bonded into complexes. Plasma transglutaminase, activated by thrombin, also cross-links external fibronectin into high-molecular-weight covalent complexes. In cultures of normal fibroblasts, pericellular matrix fibronectin displays extensive codistribution with (pro)collagens types I and III. Transformed adherent cells show decreased formation of the fibronectin-collagen matrix. The deficient synthesis of fibronectin and other matrix components and abnormal interactions with the matrix may account for several phenotypic characteristics of transformed cells. The pericellular matrix structure has been prepared by use of deoxycholate and hypotonic medium to solubilize the cells. The matrix contains glycosaminoglycans, procollagens, and fibronectin. The fibronectin codistributes with the procollagens. The matrix may be considered to be an in vitro equivalent of the connective tissue matrix and basal laminae found in vivo. Human sarcoma cells spread rapidly on the prepared matrix and assume an elongated morphology characteristic of normal fibroblasts. The prepared matrix may provide a general tool to study the effects of matrix on cellular behavior and differentiation.