A case of double neutrophilic population in a more or less symptomless patient is described. The diagnosis is made on one hand by the presence of 2 distinct neutrophil clusters on the PEROX display of the H* 1 hematological automate, on the other hand by cytochemical peroxidase staining which clearly confirms the existence of two populations. One of the populations shows normal MPO activity, whereas the second one is characterized by partial MPO deficiency.