The Technicon H6000 system simultaneously determines blood cell and leucocyte differential counts automatically using cytochemistry and cell size measurements. Part of the information provided by this instrument is a plot of peroxidase staining intensity against leucocyte cell size called 'the leucocyte peroxidase distribution display'. Recently, an abnormal leucocyte peroxidase distribution display was observed, on a sample from a post-operative lung cancer patient, in which eosinophilic leucocytes did not react with peroxidase. Neutrophils reacted in a normal way with this sample. The eosinophils were further examined by electron microscopy and manual peroxidase staining. Some peroxidase positive granules were observed in the eosinophilic leucocytes. The abnormal leucocyte peroxidase distribution display was found to be due to a separate group of eosinophilic leucocytes with lowered peroxidase activity.