Conjugate molecule 2, in which theophylline and a 4-azidobenzoyl group are separated by a short spacer chain, was adsorbed onto polyurethane sheets and subsequently irradiated with ultraviolet light. The resulting photoreaction at the polymer surface led to immobilization of theophylline. The modified polymer surface thus obtained was subjected to different physico-chemical experiments, i.e., X-ray photoelectron spectroscopy, attenuated total reflection infrared spectroscopy, and water contact-angle measurements. These experiments clearly pointed out that theophylline is linked to and exposed at the surface of the modified polymer. In addition, the modified surface was subjected to various in vitro biochemical tests, i.e. (i) a thrombogenicity assay in which the surface was contacted with either platelet-rich blood plasma (PRP) or platelet-free blood plasma (PFP), and formation of thrombin was monitored as a function of time and (ii) studies focussed on adhesion of blood platelets to the surface, both in a static system, and under flow conditions, The latter experiments were conducted with a parallel-plate flow chamber. The biochemical rests revealed that the modification leads to a marked increase of the lag-time for surface-induced thrombin formation provided that PRP was used in the test (experimental lag-times for the modified surface and the untreated control : ca. 1270 and 576 s, respectively). Using PFP, the fag-times of the modified and untreated surfaces do not differ substantially (698 and 549 s, respectively), Studying adhesion of blood platelets in a static set-up, the modified surface showed adhesion of only few platelets with nearly unchanged morphology, whereas extensive adsorption of activated platelets (evident from spreading and formation of pseudopods) was found for the untreated control surface. These observations were made with scanning electron microscopy. The experiments with the parallel-plate flow chamber showed adhesion of approximately 30.000 platelets per square cm of the modified surface, for shear rates in the range 12.5-300 s(-1). The density of adhered platelets for the untreated surface was found to be higher by one order of magnitude.