It has been reported that the 'systemic inflammatory' response occurring during cardiopulmonary bypass (CPB) may be responsible for postoperative complications. This inflammatory response is triggered by the platelet coagulation system, complement activation, resulting in neutrophil activation and degranulation. The aim of this study was to quantify the specific markers of neutrophil activation in the adult and paediatric patient during CPB. We report that neutrophil myeloperoxidase (MPO) and human neutrophil elastase (HNE) are significantly increased in both the paediatric and adult patients. This study utilized enzymatic assays that quantified the enzymatic activity in the blood circulation rather than measurement of the enzyme protein complex. Increased concentrations of MPO were, in part, related to the blood prime with the paediatric patients and the HNE concentrations were related to time on CPB in both the paediatric and adult patient. This study demonstrated that there are significantly elevated neutrophil enzyme activities in the CPB patient and underscores the pivotal role of the neutrophil in the pathophysiology of CPB.