Background: Activation of neutrophil adhesion molecules and subsequent neutrophil adhesion to vascular endothelium are key events initiating inflammatory organ dysfunction after cardiopulmonary bypass and ischemic reperfusion, Objectives: We sought to characterize neutrophil integrin CD11b and L-selectin activation associated with coronary artery bypass graft surgery and to determine whether neutrophil activation contributes to their sequestration on postbypass reperfusion, Methods: Twenty patients undergoing routine coronary artery bypass were studied, Heparinized whole blood was simultaneously sampled from a central venous line, aorta, coronary sinus, and right and left atrium before, during, and up to 20 minutes after cardiopulmonary bypass, Neutrophil counts were obtained, and neutrophil CD11b and L-selectin expression was determined by flow cytometric analysis in whole blood. Results: CD11b expression on circulating neutrophils increased during cardiopulmonary by-pass, peaking at 145% of baseline level after release of the aortic clamp and then declined by 20 minutes after bypass (analysis of variance, P = .003). No change in neutrophil L-selectin expression was observed during cardiopulmonary bypass. Neutrophils responded to ex vivo stimulation by C5a and leukotriene B-4 during cardiopulmonary bypass but not at 24 hours after the operation. After reperfusion, neutrophil loss, but not local activation, was demonstrated in the coronary and pulmonary circulations. Conclusions: Upregulated CD11b expression on neutrophils is likely to contribute to neutrophil sequestration in the heart and lungs after bypass, but neutrophil activation map be Limited by their reduced responsiveness to agonist stimulation, CD11b represents a potential therapeutic target for diminishing inflammation after cardiac operations.