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A reduction in host defence capability, with increased risks of infection, occurs after open-heart surgery to a greater degree than after nonbypass surgery. Our costs for postoperative infections are large, about 15% of the total cost of open-heart surgery; better understanding of factors contributing to infection might allow reduction in these costs. Disturbance of the mechanisms involved in the handling of haemoglobin and iron may be among such factors, since free iron is a facultative requirement for the growth of all micro-organisms. We evaluated the change in serum proteins, particularly haptoglobin, during cardiopulmonary bypass. This study, in eight patients undergoing uncomplicated aortocoronary bypass employing a bubble oxygenator with haemodilution, and without clinical evidence of haemolysis, showed that free, unbound haptoglobin, evaluated by agarose gel electrophoresis, disappeared and was replaced by haemoglobin-bound haptoglobin within minutes of going on bypass. Such changes were not seen in three control patients who underwent comparably extensive surgery without bypass or in-bypass controls, in whom blood was evaluated after dilution with oxygenator prime. The disappearance of free haptoglobin and the possible disturbance of iron transport may have implications for host defences after bypass. [ABSTRACT FROM PUBLISHER]