Blood and/or bone marrow cells from patients with hematological malignancies, chronic lymphocytic leukemia (CLL), chronic myeloid leukemia in blastic phase, acute lymphoid leukemia, acute myeloid leukemia, prolymphocytic leukemia, multiple myeloma and myelofibrosis, were incubated with Lonidamine at different concentrations (50, 100 and 160 micrograms/ml) for 1-2 h. B lymphocytes of CLL had the highest sensitivity to the drug: a decrease of more than 30% in the viable nucleated cells was recorded in 8 out of 10 experiments. Subsequent in vivo studies were performed to investigate the effectiveness of Lonidamine in the treatment of CLL. The drug was administered orally for a period of 7 days to 19 selected patients, both previously treated and untreated, in different stages of the disease. Only 5 patients responded: 2 of them had a significant decrease (greater than 30%) in the lymphocyte count, and the remaining 3 showed an appreciable reduction of the spleen (1 case) and of the lymph node size (2 cases). Generalized myalgia was the most common side effect induced by Lonidamine.