The correct diagnosis and classification of the acute leukemias relies very often on the accurate recognition and characterization of the blast cell population. Romanowsky-stained blood and bone marrow films permit this in most cases if the features of differentiation are well developed. When this is not so, cytochemical methods may demonstrate features not apparent on conventional stains, thus providing an indication of the degree of cell maturation. An important step towards a uniform nomenclature and a more objective classification was provided by the joint proposals of the FAB group [1]. The FAB workers recognized two main groups of acute leukemia: those showing myeloid differentiation (AML) and the “lymphoblastic” leukemias (ALBL). Most categories of AML (M2, M3, M4, M6) can be recognized on Romanowsky-stained films alone. For Ml and M5, in which signs of maturation of the blast cells are less obvious, cytochemical tests may be essential.