Heparin adsorption onto polyethylene imine (PEI) films was investigated. The adsorption was followed with time-dependent surface plasmon resonance (SPR) for PEI cast on gold films, and the thickness of the adsorbed heparin was determined by fitting the angular reflection curves to Fresnel’s equations. The average thickness of the PEI determined the heparin adsorption range and it was found that both heparin binding capacity and sensitivity increased with PEI thickness. At an appropriate thickness of 10 +/- 2 nm, adsorption of heparin in the clinically applicable range of 0-2 U/ml caused a linear concentration dependent change in the SPR reflectivity at a fixed angle, with excellent sensitivity (0.05 U/ml). The results are compared to heparin adsorption on immobilized protamine and the surface coverage and probable heparin distribution discussed.