In patients undergoing cardiopulmonary bypass (CPB), red cell damage was assessed by simultaneous measurement of of plasma-haemoglobin (P-Hb) and red cell filtration rate (RFR) (1). RFR was significantly reduced after 120 min as compared with 30 min (17.4±4.1 vs. 30.4±2.9 μl/s). P-Hb, by contrast, was significantly higher at 60 min than at 30 min after start of CPB (195±27.2 vs. 73.8±8.1 μl/s). Significant negative correlation was found between RFR and P-Hb (r=0.68). Red cell deformability thus permitted qualitative assessment of red cell trauma during CPB and thereby an estimation of the rheologic disturbance caused to these cells by CPB during open-heart surgery.