We investigated changes in quality of plasma fibronectin with aging, especially in the interaction of fibronectin with collagen, using an enzyme-linked immunosorbent assay. When fibronectin isolated from the plasma of young people was compared with that from old people, the latter was found to bind poorly to gelatin, Type I and Type III collagens. No differences in molecular weight were detected between the two types of fibronectin by SDS-polyacrylamide gel electrophoresis