Reduced blood cell rheology due to mechanical blood trauma is associated with reduced microcirculation and organ dysfunction during and following cardiac surgery using cardiopulmonary bypass (CPB). The influence of prostaglandin E 1 (PGE 1 ) on the blood cell rheology was studied in 24 patients undergoing coronary artery bypass grafting surgery with the use of CPB. Intra-operative infusion of PGE 1 was given to 12 patients in a dose of 20ng/kg/min; the other 12 patients served as controls. Red and white cell rheology were measured by red and white cell filtration rate (RFR and WFR) respectively, applying a standard bulk filtration technique. Blood samples were collected pre-operatively, at the start of CPB, after 30 minutes, 60 minutes, 90 minutes and at the end of CPB. Results showed that the mechanical trauma of CPB was better tolerated in the PGE 1 group, where RFR and WFR showed significant lower losses during CPB (p < 0.001, p < 0.001). These results indicate that PGE 1 has beneficial effects on red and white cell deformability during CPB. The improvement of blood rheology would be expected to lead to better microcirculation in the vital organs.