The adsorption of a synthetic heparinoid polyelectrolyte on poly (vinylchloride) previously treated with tridodecylmethylammonium chloride (TDMAC) was studied using radiotracer techniques to provide a more quantitative picture of antithrombogenic surface coatings. 125I-labeled polyelectrolyte was used for overall adsorption measurements. Experiments with 36Cl-labeled TDMAC yielded information on actually adsorbed polyelectrolyte segments in good agreement with existing theory. A large excess of free dangling segments was found. In the presence of NaC1 the total adsorption was diminished, although no desorption occurred on treatment of adsorbates with 0.15 M NaC1 or blood plasma.