This study was designed to compare the effects of three prolonged exercises on plasma myeloperoxidase concentrations, used as marker of neutrophil degranulation. Seven healthy subjects underwent three effort trials (45% VO2max during 4 h, 60% VO2max during 3 h, 75% VO2max during 2 h) at one-month intervals. Blood samples were collected before, during and after exercise. Myeloperoxidase levels were measured by radioimmunoassay. We observed a significant increase in myeloperoxidase concentration (+85%) at the end of our three prolonged exercises (p < 0.05). This myeloperoxidase increase was not correlated with the modification of neutrophil count, suggesting that neutrophil degranulation is independent of their mobilisation.