Potassium cyanide (KCN) resistant peroxidase is generally accepted by hematologists as a selective stain for the eosinophilic cell line. However, it has been demonstrated biochemically that not only the peroxidases of neutrophils but also those of eosinophils can be inhibited by KCN. Therefore, bone marrow smears of hematologically normal patients were subjected to the peroxidase reaction in the presence of varying concentrations of KCN. It was found that with increasing concentrations of KCN not only neutrophils but also eosinophils were inhibited. Moreover, there were always neutrophilic promyelocytes that were still positive when a considerable number of the eosinophils was already inhibited. Therefore, it can be concluded from our results as well as from biochemical data that there is no concentration of KCN which demonstrates the total of the eosinophilic cell line selectively. The implications of these findings are discussed.