Benfluoron (BF), a new cytostatic drug, synthesized at the Institute of Pharmacy and Biochemistry in Prague, was tested for its cytostatic and cytotoxic effect. The concentrations of BF ranging from 0.1-2 micrograms/ml had a significant cytostatic effect on nine stabilized human leukemic cell lines. This effect was demonstrated by cell counts, cell viability and 3H-thymidine incorporation. The BF concentrations of 2 micrograms/ml and higher were considerably cytostatic causing the cell death and cell degradation. The effect of BF on the cell growth being irreversible could not be eliminated by washing the cells and recultivating them in fresh medium. The BF concentrations halving the total number of viable cells (EC50 value) induced a higher cytotoxicity in lymphoblastoid cell lines then in myeloid ones. BF did not influence the binding of monoclonal antibodies with membrane markers of different cell subpopulations. Prospective application of BF as a cytostatic and immunosuppressive drug is discussed.