The effect of lonidamine on glucose metabolism, hexokinase activity and adenylate pool of MCF-7 human breast cancer cells sensitive and resistant to adriamycin has been investigated. The following summarizes the results: 1. In both cell types the greatest part of glucose was metabolized to lactate, whereas only a small proportion of glucose carbon atoms was incorporated into CO2, lipids, nucleic acids, and supporting structures. 2. Glucose utilization, lactate production, and ATP content were higher in resistant cells due to a greater activity of mitochondrial hexokinase. 3. Lonidamine decreased glucose utilization, aerobic glycolysis and ATP content in both cell types and the effect was significantly higher on resistant cells. 4. The extent of inhibition in sensitive and resistant cells overlapped that found for mitochondrially bound hexokinase, thus indicating that the greater sensitivity of resistant cells to lonidamine was due to their higher amount of bound hexokinase. These findings confirmed a modified glucose metabolism in cells with resistant phenotype and suggested that lonidamine might be usefully used to reduce or overcome multidrug resistance of those cells with a reduced ability to accumulate and retain antitumor drugs.