We previously isolated etoposide/teniposide-resistant cell lines from human cancer KB cells, designated KB/VP-2 and KB/VM-4, respectively, and we found that decreased expression of topoisomerase II alpha was associated with the acquisition of etoposide/teniposide resistance in both resistant cell lines. In this study, we studied how the expression of the DNA topoisomerase II alpha gene is regulated in drug-resistant cell lines at the transcriptional level. We first examined whether the decreased topoisomerase II alpha mRNA level was due to a shorter lifetime of mRNA molecules in drug-resistant cell lines. A comparison of the degradation kinetics of topoisomerase II alpha mRNA demonstrated that there was no difference in mRNA stability between both resistant cell lines and their parental counterpart. A run-on experiment with isolated nuclei showed that the transcriptional activity of topoisomerase II alpha gene of both resistant cell lines constituted less than 20% of the parental KB cells. The activity of DNA topoisomerase II alpha promoter in resistant cells was also less than 20% of that in KB cells when transient transfection assays were performed with the promoter-driven bacterial chloramphenicol acetyltransferase gene. Among the several transcription factors that might be involved in DNA topoisomerase II alpha gene expression, expression of Sp3, an inhibitory member of the Sp1 family, was elevated to about 3-fold higher in both resistant cell lines than their parental counterpart. These results indicated that the expression of DNA topoisomerase II alpha gene decreased at the transcriptional level through the enhanced expression of Sp3 in our two etoposide/teniposide-resistant cell lines.