Thromboembolism (valve thrombosis and systemic embolism) is the main drawback of mechanical heart valve prostheses. The patients carrying these valves have to be subjected to life-long anticoagulant therapy to reduce thromboembolism. This therapy does not completely prevent these complications and may, if not properly controlled, even lead to life-threatening bleeding problems. Hemocompatibility of a mechanical heart valve is related to its engineering design and the construction material. To improve hemocompatibility of a mechanical heart valve, not only design but also valve material must be improved. Therefore the search for new materials or surface coatings that are more hemocompatible than those currently used must continue. The purpose of the present investigation was to develop an in vivo method, and to evaluate and compare hemocompatibility of some materials currently used, and for potential use, in ·prosthetic heart valves. Pyrolytic carbon (PyC), titanium (Ti), cobalt-chromium (CC), glutaraldehyde-treated bovine pericardium (Pe) and some new materials such as titanium nitride (TiN) and diamond-like carbon (DLC) were evaluated in three series of sheep experiments. The test materials were implanted in the central veins in the first and second series, and in the descending aorta in the third series. Up to four different materials could be tested simultaneously in each animal. No anticoagulant was given. After two hours of exposure to flowing blood, the test surfaces were explanted and prepared for photography and scanning electron microscopy (SEM). Thrombus area (the area covered by thrombus) was measured on close-up photographs of each surface using planimetry. Blood cell adhesion and blood-surface interaction were observed with SEM. The results showed more thrombi on PyC and Pe than on Ti and TiN. Leukocytes were the main type of blood cells adhering to PyC and DLC, and erythrocytes to Ti and TiN. Different materials exhibited different patterns of blood-surface interaction. Thrombus composition was largely related to the pattern of cell adhesion, indicating that the mechanism of early thrombus formation might be different on different surfaces. The results suggested that the method is practical and reliable. Under the present conditions PyC was not as hemocompatible as the metals currently used. TiN was more hemocompatible than PyC. Due to its combination of excellent hemocompatibility and wear resistance, TiN may be a promising new surface coating material for metallic components of mechanical heart valves, blood pumps and other devices in contact with blood.