The accuracy of diagnosis of hematological diseases from bone marrow aspirates and peripheral blood smears is considerably improved by use of histochemical stains. As hematopoietic cells differentiate and mature from the pluripotent stem cell and are committed to a particular cell line, the committed cell lines produce different substances, particularly enzymes, which can be identified by various histochemical stains. Histochemical stains are not diagnostic, but are aids in diagnosis, because various cell lines may produce similar substances and because the stains lack absolute specificity. A discussion of the use of histochemical stains for the diagnosis of leukemias is presented. Complete procedures for Prussian blue stain for iron, peroxidase, Sudan black B, naphthol AS-D chloroacetate esterase, nonspecific esterase, periodic acid Schiff (PAS), acid phosphatase, toluidine blue and alkaline phosphatase stains are given in the appendix.