In the attempts to classify leukemic cells it is often difficult to identify cells solely with fixed and stained blood smears as in acute leukemia where the percentage of blast cells is high. In such instances, however, it is generally possible to distinguish most cells when vital observations are carried out in conjunction with cytochemical methods, fluorescence micro scopy, as well as electronmicroscopy. The results of these observations have been briefly summarized. The characteristic findings of lymphoblasts, myeloblasts, and monoblasts as revealed by May-Giemsa staining, brilliant cresyl blue supravital staining, phase-contrast microscopy, vital staining and electronmicroscopy. are illustrated in the tables.