The term biocompatibility may he broadly defined as the ability of a material to perform in vivo without inducing a clinically significant host response due to contact with body fluids or tissue. Few methods are able to characterize, in real-time, the acute inflammatory response under physiologic conditions. As LVAD's and TAH's become more sophisticated, it seems logical to assume that control algorithms using feedback from minimally invasive sensors that monitor the state of tissue perfusion may become a reality. Our research has focused on the development of small needle like biosensors for minimally invasive applications. These sensors require the application of a diffusion-limiting, polymer membrane. One of the most serious drawbacks to long-term use of such sensors is the issue of biofouling of polymer membranes and its deleterious effects on sensor output. An in vitro method, using physiologic fluids, has been developed to simulate and measure, in real time, the response of a working biosensor at each stage of the process of protein adsorption, cell binding and thrombus formation. Results will he presented for a number of commercially available biopolymers including segmented aliphatic and aromatic polyurethanes and hydrophilic polymers commonly used in the construction of diaphragms for artificial hearts. Based on the results, an index of thrombogenicity was developed. (a) Williams, DF, Med Prog Techno/, 4, 31–42 (1976); (b) Curland HJ, Nephrol Dial Transplant, 9, 4–10 (1994).