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Scanning electron microscopic observation of blood samples before, during and after cardiopulmonary bypass during cardiovascular surgery revealed that 4 to 25% of the red blood cells undergo a progressive transformation of discocyte to echinocyte. A morphological index, I, was developed and the change in I (Ir) was found to correlate well with measurements of free plasma hemoglobin. Incubation of blood samples form normal subjects for 90 minutes at 37°C resulted in no increase in Ir whereas incubated samples from patients following cardiopulmonary bypass showed an increased Ir to a mean value of 1140 ±185. Incubation therefore appears to uncover sublethal red cell damage caused by extracorporeal circulation. It is suggested that this technique is a sensitive index of red cell trauma which may have useful clinical applications.