Camptothecin-11 (CPT-11) is a new derivative of camptothecin, a plant alkaloid antitumor agent. Previous studies indicated that antitumor activity of CPT-11 was mediated through interaction of the drug with its target enzyme, DNA topoisomerase I (topo I). To elucidate the mechanisms of CPT-11 resistance, we have characterized glioma cell lines (T98G/CPT-11, C6/CPT-11) selected from the wild types (T98G. C6) for acquired resistance to CPT-11. T98G/CPT-11 and C6/CPT-11 cells demonstrated 5.4- and 7.3-fold increases, respectively, in resistance to CPT-11. Total glutathione S-transferase (GST) and GST-p activities were similar in CPT-11-sensitive and -resistant cells. No difference in intracellular accumulation of CPT-11 was observed between CPT-11-resistant and parental cells, indicating that an alteration in the uptake was not responsible for resistance. In addition, CPT-11-resistant cell lines showed no change in the total activity of Topo I, indicating an alteration in total Topo I was not responsible for resistance. In contrast, significantly increased intracellular glutathione (GSH) levels levels were found in T98G/CPT-11 and C6/CPT-11 cells (4.3- and 2.1-fold). Furthermore, Topo I samples from T98G/CPT-11 and C6/CPT-11 cells were at least 4- and 2-fold more resistant to the inhibitory effect of the CPT-11 on the relaxation activity of Topo I than were Topo I samples from their respective parent lines. The resistance of the enzyme itself to the effects of CPT-11 may be responsible for the resistance to CPT-11. Thus, at least two distinct mechanisms have been selected for the CPT-11-resistant cells.