A major problem in cancer treatment is the progressive desensitization of the cancer cells to chemotherapeutic drugs. Several hypotheses have been advanced to explain this property of neoplastic cells. In recent years, some calcium-channel blockers have successfully been used to restore drug-sensitivity in previously resistant tumors. The presence of a correlation between ion channels and membrane fluidity is well known. In the ambit of our studies on the activity of several chemotherapeutic drugs on tumors, we have studied the variations in membrane depolarization and fluidity in some leukemic cells as a result of polychemotherapeutic treatments. Our results demonstrate that the membrane fluidity and K(+)-induced depolarization of some types of leukemic cells in patients untreated and treated with some chemotherapeutic agents, are altered significantly as compared to those of normal leukocytes.