X-ray repair cross-complementing gene I protein (XRCC1) in complex with DNA polymerase beta, DNA ligase III, and poly(ADP-ribose) polymerase is important in the base excision repair process. Previously, we isolated camptothecin (CPT)-resistant cell lines (KB100 and KB300) from the human epidermoid carcinoma cell line KB by exposure to CPT. From these CPT-resistant cell lines, their revertants (KB100(rev) and KB300(rev)), which lost most of their CPT-resistant phenotype during passage in the absence of CPT, were established. In this study, we found the expression levels of XRCC1 protein in KB100 and KB300 were > or =5-fold more than in their respective revertant cell lines, whereas there was no difference in the expression of XRCC1-associated proteins such as DNA polymerase beta, DNA ligase III, poly(ADP-ribose) polymerase, and apurinic/apyrimidinic endonuclease. The degree of CPT resistance was relatively correlated with the XRCC1 protein amount. We also found XRCC1 gene amplification in CPT-resistant KB100 and KB300 cell lines. To confirm a correlation between overexpression of XRCC1 and CPT resistance, we transfected the XRCC1 gene into KB100(rev) and obtained two different transfected cell lines (clones 14 and 16). The expression levels of XRCC1 in the transfected cell lines were higher than in KB100(rev) but lower than in KB100 with no difference in XRCC1-associated protein expression levels. Resistance to CPT in transfected cell lines was 2-2.5-fold higher than in KB100(rev) in regard to growth inhibition and 4-fold higher with respect to clonogenicity. Transfected cell lines also showed increased resistance to other topoisomerase I poisons. However, the cytotoxicity of VP-16 and cisplatin was similar in both the transfected cells and KB100(rev). Similar to our CPT-resistant cell lines, the resistance of transfected cell lines was reversed by treatment with 3-aminobenzamide. These results indicate that CPT resistance in our cells could be partly attributable to the overexpression of XRCC1.