A case of acute myeloid leukaemia (AML) (M2 type) is described in which the neutrophil population contained markedly increased peroxidase activity. The increased peroxidase activity was initially detected during routine counting with the Hemalog D automated differential counter and the increased enzyme staining was confirmed by manual and ultrastructural cytochemistry. The abnormal neutrophil population persisted after blast cells had disappeared from the blood and until marrow hypoplasia was induced by chemotherapy. When remission emerged, the peroxidase activity of the neutrophil population was normal. At subsequent relapse, the neutrophil population with increased peroxidase activity reappeared and with the induction of second remission neutrophil peroxidase again returned to normal. In 8 other cases where sufficient neutrophils were present for analysis by the Hemalog D, neutrophil peroxidase was normal or not increased in 7 cases, but was markedly reduced in 1 case of AML (M6 type). It is suggested that abnormalities of neutrophil peroxidase activity detected by the Hemalog D may serve as useful leukaemic markers.