Human tumor degenerating factor (TDF) activity was inhibited by the addition of more than 8 micrograms/ml of fibronectin. The crude TDF preparation also inhibited the activity of TDF. The inhibitor of TDF activity was isolated from the crude TDF preparation by gelatin-Sepharose chromatography. Furthermore, this inhibitor was isolated from the fetal bovine serum (FBS), which is used for the production of TDF, by the same chromatography. Since the inhibitors from TDF preparation and the FBS showed the cross reaction with fibronectin by micro-Ouchterlony using anti-fibronectin serum, it was suggested that a part of the inhibitors was identical to fibronectin.