The influence of extracorporeal circulation on erythrocytes and flow properties of blood. The influence of extracorporeal circulation on red blood cells and flow properties of blood was studied in 10 patients undergoing aorta-coronary bypass grafting. Blood samples were drawn on admission, under general anesthesia before the operation, during extracorporeal circulation, immediately after extracorporeal circulation, and 24 hours after extracorporeal circulation. Echinocytes were found during and shortly after extracorporeal circulation, but disappeared within 24 hours. Washing the cells in buffer restored the normal discocytic shape, which indicated that a plasma factor was responsible. Red cell membrane lipids were not affected. Analysis of the membrane proteins revealed a decrease of ankyrin after extracorporeal circulation, which was prevented by protease inhibitors during preparation. This suggests an increased proteolytic activity of the plasma after extracorporeal circulation. Red cell deformability was not altered. Plasma viscosity and hematocrit were markedly reduced by hemodilution with the priming solution. Their low levels resulted in a low blood viscosity during extracorporeal circulation, which was even lower at 26 degrees C than before or after the operation at 37 degrees C. We conclude that the red cell is affected by extracorporeal circulation. The flow properties of blood, however, are not impaired, but are improved by hemodilution. 