The effect of hydroxyethylrutosides (HR) on erythrocyte deformability was studied in 13 adult patients subjected to extracorporeal circulation, in seven cases for single valve replacement and in six for coronary bypass operations. A single dose of 1.5 g HR was given by slow intravenous injection immediately before the cardiopulmonary bypass. The controls were 13 patients undergoing the same operations but without HR. In the HR-medicated valve group there was only 3% decrease in erythrocyte deformability following extracorporeal circulation, in contrast to a 41% (p less than 0.01) decrease in the control valve group. Among the coronary patients there was no such difference between the HR and the control groups, with deformability decreasing by 21 and 26%, respectively (both significant, p less than 0.05). HR administered before extracorporeal circulation thus had significant prophylactic effect on red cell deformability in patients undergoing valve replacement. Such beneficial action may improve nutritional blood flow, thereby reducing the number of postoperative complications in various organs. With higher doses and/or longer periods of administration, a favorable effect of HR might be possible also in patients subjected to coronary surgery.