The specificity and the clinical usefulness of the hybridoma derived monoclonal antibodies raised against the differentiation antigens of granulocytes (VIM-D5, VIM-C6), monocytes (VIM-D2), B lymphocytes (VIB-C5, VIC-Y1), erythrocytes (VIE-G4) and CALLA (VIL-A1) was studied in leukemic cells isolated from peripheral blood and bone marrow of 41 adults with acute leukemia by using indirect fluorescence method. The VIL-A1 positivity was observed in 4/9 of ALL and in none of myeloid leukemia. It was accompanied in 3/4 of cases by VIB-C5 positivity and in one case by VIE-G4 positivity. It is important that 2 out 3 unclassifiable cases (PAS-) could be diagnosed as common ALL due to their VIL-A1 positivity. VIM-D5 like VIM-C6 reacted specifially with granulocytic cells only and gave positive results in 20/30 of acute myeloid leukemias. When classified according to the FAB scheme, the proportion of VIM-D5 + patients rose from M1 toward more mature subtypes, including M4/5. It allowed to identify as AML 2/6 of unclassifiable-Mo leukemias, VIC-Y1 appeared to be helpful in characterization of B cell malignancies and of myelomonocytic leukemias. It is concluded that the monoclonal antibodies of VI series are specific and allow a more precise definition of leukemia, thus helping in optimalization of treatment.