In vivo long-term stability of intraocular lens surfaces, modified by immobilized heparin, was studied. Lenses were implanted in the anterior chamber of rabbits and analyzed for surface concentration of heparin after varying periods of time up to two years. A new method using adsorption of 125iodine-labeled protamine was developed for quantitative measurements of immobilized heparin. Another assay, based on a monoclonal antibody against heparin, was also included. The study showed that surface-immobilized heparin did not degrade or desorb to any measurable degree during a two-year follow-up.