AbstractPoly(sodium vinyl sulfonate) (PVS) was found to be 1/14 times as active as heparin in inducing the conformational change and activation of antithrombin III. The conformational change of antithrombin III was investigated in terms of the intrinsic fluorescence of tryptophan residue, the extrinsic fluorescence using 1,6‐diphenyl‐1,3,5‐hexatriene as fluorescent probe, and Fourier‐transform infrared spectroscopy. It was evident in the experiment using 2,4,6‐trinitrobenzene sulfonate that PVS elicited the activity of antithrombin III by interacting with amino groups of the protein as does heparin.Sodium vinyl sulfonate was graft‐polymerized onto polyetherurethane (PEU) film that was treated with glow discharge in advance. PVS‐grafted PEU film adsorbed antithrombin III easily and, like ungrafted PVS, induced conformational change and activation of antithrombin III. However, the mechanism of interaction of the PVS graft with antithrombin III did not seem to be completely the same as that of ungrafted PV