Decreased complement levels and impairment of polymorphonuclear leukocyte function increase the risk of infection during cardiopulmonary bypass (CPB). The effects of different types of oxygenator and of blood suction on this natural humoral and cellular host defense mechanism were investigated in dogs undergoing CPB during sham open-heart operations. Airborne contamination of the wound area and the CPB circuit was performed by aerosolizing Staphylococcus aureus. A membrane oxygenator in the CPB circuit maintained a normal host defense mechanism. The use of cardiotomy suction during CPB with this type of oxygenator affected the host defense to some extent. The use of a bubble oxygenator in the CPB circuit together with cardiotomy suction seriously impaired the host defense. Postoperatively bacteremia developed in no dogs in the membrane oxygenator group, whereas 8 of 15 dogs in the bubble oxygenator group had a positive blood culture for the indicator microorganism. We conclude that the use of a membrane oxygenator is helpful to maintain the host defense. Attention has to be paid to reduce the deleterious effects of cardiotomy suction.