The effect of Rhein (RH) and Lonidamine (LND) on the clonogenic activity of cultured human glioma cells has been evaluated. Both these drugs decrease the survival fraction, but their effect is strictly related to the duration of exposure. A brief exposure, i.e. 4 hours, even at the highest drug concentrations does not induce any significant decrease in the survival which, on the contrary, is strongly affected by 24 and 48 hours of exposure. The reason for this behaviour lies in the mechanism of action of these drugs which do not interfere with replicative processes, but selectively affect the energy metabolism of the neoplastic cell. The validity of currently employed screening tests to evaluate the antitumoral activity of non anti-mitotic drugs is also discussed.