UNLABELLED The hemocompatibility of a new hollow fiber membrane oxygenator, BOS- CM40 , was evaluated during and after cardiopulmonary bypass for coronary artery bypass graft operations in 10 patients. Blood cells were well preserved by this oxygenator. In particular, platelet numbers at the end of bypass were significantly higher than at the start of bypass and platelet function remained completely intact during bypass. After protamine hydrochloride administration, platelet function decreased slightly but platelet numbers remained unaltered. After release of the aortic cross-clamp the beta-thromboglobulin concentration sharply increased, which shows the damaging effect of cardiotomy suction. Nevertheless, platelet numbers and function did not decrease significantly during this period. Apparently, platelet number and function can remain unaffected by this damaging procedure, provided they are normal before the start of suction. The average postoperative blood loss was 551 ml, which appears to be less than the blood loss in a previous study in which a membrane oxygenator of the envelope type was used. (J. Thorac . Cardiovasc . Surg . 83 (1983) 108-116). IN CONCLUSION The BOS- CM40 hollow fiber membrane oxygenator proved to be highly hemocompatible .