Complement plays a vital role in the body's defence systems. Cardiopulmonary bypass induces a detrimental inflammatory reaction in which the complement system is known to participate through direct effects as well as through activation of neutrophils, platelets and endothelial cells. On the other hand, it has been suggested that in the setting of cardiopulmonary bypass, complement may be activated by neutrophils, perhaps due to fragmentation caused by the heart-lung machine. We therefore investigated whether intact or fragmented neutrophils were able to activate the complement system, and whether neutrophil-platelet interaction could influence such complement activation. Lepirudin-anticoagulated plasma was incubated at 37 degrees C with resting or activated intact neutrophils or neutrophils combined with platelets, or increasing amounts of fragmented neutrophils. Complement activation was evaluated by measurement of C1rs-C1 inhibitor complexes, C4bc, C3bBbP, C3bc, C5a and sC5b-9. We found significant activation of complement only by unphysiological doses of fragmented neutrophils or supernatant from fragmented neutrophils, consistent with a limited clinical significance related to neutrophil destruction during cardiopulmonary bypass. Unstimulated neutrophils induced C3bPBb formation but little formation of other activation products, indicating an increased C3 hydrolysis which was kept under control by regulatory mechanisms. Neutrophils and platelets combined increased classical activation and decreased alternative activation, similar to the findings with platelets alone. Our data confirm that in the setting of acute neutrophil fragmentation or activation, complement activation is much more important in the inflammatory network as an event upstream to neutrophil activation than vice versa.