Fibronectins isolated from culture media conditioned by the growth of early (young) and late (old) passage human fibroblasts were compared by affinity chromatography and polyacrylamide gel electrophoresis. The results indicated that fibronectins from young and old cells were similar, but not identical. The fibronectin synthesized by old cells migrated more slowly on NaDodSO4 polyacrylamide gels in the presence of reducing agent than did fibronectin from young cells. The apparent molecular weight difference for purified fibronectins compared on gradient gels was estimated to be 5-10 000 daltons. The molecular weight difference was also evident in unpurified fibronectins in whole conditioned media. Both young and old fibronectins formed disulfide bonded dimers in the absence of reducing agents indicating that the molecular weight difference was not generated by proteolytic cleavage at the C-terminus of the molecule. Further, both fibronectins were bound by heparin-Sepharose, thiol-activated-Sepharose, and gelatin-Sepharose resins. Comparison of peptide maps, generated by limited proteolytic digestion revealed several differences. In particular, a polypeptide of molecular weight approx. 160 000 was larger in old cell fibronectin than in young cell fibronectin. This polypeptide had heparin binding activity, but lacked affinity for gelatin.