The most critical parts of those medical devices that come in contact with blood or other tissue are biomedical materials. The performance of devices such as indwelling catheters, heart valves, and haemodialysers attest to this vital area. However, with the possible exception of medical grade silicone rubber (Dow Corning Corp.), almost all other polymers are common industrial products not specifically developed for biomedical applications. These invariably contain fillers, traces of catalysts, and stabilizers. Examples are plasticized poly(vinyl chloride) (PVC), Teflon@ polytetrafluoroethylene, polyethylene, polypropylene, Dacron@ polyester, nylons, and polyurethanes. None of them is thromboresistant. The most widely used polymer in medical applications is plasticized PVC even though large quantities of the plasticizer 2-diethylhexyl phthalate (DEHP) leach out into blood’. Although DEHP seems to have low acute toxicity, its long-term effects, including carcinogenicity, are still being debated2*. In any case, the large quantities of DEHP leaching out into patients represent added burden on the liver and it is detrimental. Furthermore, one of the hydrolytic degradation products of DEHP is monoethylhexyl phthalate (MEHP) that is a highly toxic metabolite5. Since the Medical Devices Act of 1974 empowers the Food and Drug Administration to regulate medical devices rather than biomedical materials, it is not surprising that the materials aspects of many medical devices have received less attention and are often buried under the umbrella of ‘device engineering’. Nevertheless, the problems are significant and the evaluation of polymeric materials, especially those in contact with whole blood and its components, deserves detailed scrutiny. Although the determination of physico-chemical properties of materials represents an important task, even more essential is biological testing of well-characterized materials using first in vitro procedures and subsequently appropriate animals. All too often, however, experimental animals are used unnecessarily. Well selected in vitro tests must precede the use of animals and the latter should be treated according to the guidelines that state, among others? (a) ‘the research should be such as to yield fruitful results for the good of society, not feasible by other methods or means of study, and not random and necessary in nature’; and (b) ‘the experiment should be so designed and based on knowledge of the disease or problem under study that the anticipated results will justify its performance’. No single in vitro, ex vivo, or in vivo test can predict reliably the biological performance of biomedical materials and the term ‘thromboresistance’ is only a relative one in its meaning because no materials in contact with blood are fully compatible with it7t8. Normal endothelial cells maintain active secretory processes that not only prevent platelet adhesion and activation but are also capable of dissolving small emboli. Furthermore, in contrast to most synthetic materials, there is no evidence that the living, healthy endothelium adsorbs proteins from circulating blood. Consequently, attempts to explain thromboresistance mainly in terms of adsorption, desorption, and turnover of specific plasma proteins (especially fibrinogen, albumin, and globulins) often resulted in futility despite the elegance of the experiments. The clinical uses of blood-contacting materials still require constant pharmacological interventions, including heparinization of the patients. Such measures are necessary, for example, during haemodialysis, haemofiltration, cardiopulmonary bypass, and mechanical left ventricular assistance, in which the patient’s blood is exposed to lengthy plastic tubings (mostly plasticized PVC) and syntheticdialysis membranes. Species-related haemotological differences between experimental animals are often treated with considerable ignorance when it comes to testing of biomedical materials and devices. Biological test procedures, whether in vitro, ex vivo, or in vivo, are all too often carried out with animals whose haemotological profiles differ significantly from those of humans. Although dogs and calves have been used most frequently, the blood of other animals including rats, mice, cats, pigs, lambs, and sheep have their champions. It is frequently impossible to make valid comparisons and conclusions on the performance of many biomedical materials. In other cases where experimental work was done with a single animal species such as calves, these animals are often chosen on the basis of cost, case of handling, and physiological and anatomic considerations. Such experiments may yield useful data on the physiological performance of certain devices, however, there remains the problem of evaluating properly the performance of their component materials in contact with blood. Biomedical materials can be tested in many cases in vitro with properly screened, fresh, donated human blood. Even if donated human blood is not available, candidate biomedical materials should be tested with animals whose haemotological profiles and responses closely resemble those of humans.