Fibronectin is generally lost from virally induced tumors in vitro. In contrast, human malignancies in vitro may or may not possess fibronectin on their surfaces; examples of each have been reported. Since the cells are grown in serum containing fibronectin, the distinction between adsorbed and synthesized fibronectin must be made. Metabolic incorporation of radio-labelled precursors indicates synthesis and binding generally correlate well; non-specific adsorption of bovine fibronectin does not appear to occur; thus, loss of fibronectin from the cell surface does not uniformly occur in human tumors in vitro.