Aprotinin is a naturally occurring serine protease inhibitor that is being used with increasing frequency in cardiac surgery to reduce blood loss and the need for perioperative blood transfusion. Through inhibition of serine proteases such as plasmin, aprotinin significantly reduces fibrinolysis, thereby aiding hemostasis during surgical procedures. These proven benefits are supplemented by the anti-inflammatory properties of aprotinin, which may help curb some of the deleterious effects of cardiopulmonary bypass. Two aprotinin dosing regimens are used for prophylactic reduction of perioperative blood loss and the need for blood transfusion in patients undergoing cardiopulmonary bypass (CPB). Serum concentrations achieved with the full-dose regimen inhibit both kallikrein and plasmin activity resulting in attenuation of the systemic inflammatory response to bypass, whereas serum concentrations achieved with the half-dose regimen only inhibit plasmin activity.