Methods devised in the past to evaluate blood-material interactions in extracorporeal devices assess small segments of test materials and do not analyze the entire circuit. In order to develop an effective method for the control of thrombosis in extracorporeal circulation, it is important to perform a quantitative evaluation of random samples of all the circuit elements. In an effort to quantify thrombosis in experimental extracorporeal membrane oxygenation circuits, random samples of silicone rubber membrane and polyvinyl chloride tubing were examined using scanning electron microscopy (SEM) and quantitative morphometry. The results indicate no statistically significant differences in the amounts of various adhered blood components between the tubing and the membrane. Greater than 60% of the examined surface had no blood components present. Platelet adhesion on the surface was predominant. Evidence of fibrin, gross thrombi, red cells, and white cells in small quantities were noted. It is concluded that quantitative scanning electron microscopy (QSEM) not only provides accurate information about the morphologic changes of blood cells due to surface contact, but also quantifies the individual elements and therefore assesses their contributions to thrombosis. This quantitative description will allow more meaningful comparison of blood-compatible materials and surfaces.