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 microliters/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 microliters/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.