Phagocyte released reactive oxygen species are often discussed in connection with ischemic and reperfusion injuries to the myocardium. The kinetics of the accumulation and oxidative burst of human blood phagocytes was studied by chemiluminescence during open heart surgery in the myocardium of human patients. Direct evidence is presented for an accumulation of neutrophils along with their markedly increased metabolic activity (oxygen radical formation), especially following the reperfusion of the ischemic myocardium. Leukocyte numbers and activity remained significantly elevated even in the venous blood obtained 24 h after the operation.