Red cel survival was prospectively studied, using Chromium-51 isotope, in 48 patients undergoing cardiopulmonary bypass for single valve replacement (SVR, 24 patients) or coronary bypass graft procedures (CABG, 24 patients). The red cells were labelled on the day before the open heart surgery. Cells from randomly selected patients were treated for 30 min with potassium cyanate (0.5 mg/100 ml) in 5% invertose solution or with 5% invertose alone. The mean red cell survival in the SVR and the CABG group was 21.3 and 23 days, respectively, when the cells had been treated with cyanate, and 12.7 and 12 days when only invertose had been used. In the cyanate groups the loss of isotope during cardiopulmonary bypass and in the first three postoperative days was significantly less than in the control groups indicating improved tolerance to the traumatic effects of cardiopulmonary bypass.