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Abstract: Global DNA hypomethylation in tumor tissue is a common characteristic in a variety of malignancies such as breast, colon, oral, lung, and blood cancers. A rapid and sensitive method has been developed for the determination of global DNA methylation in cells. Five substances—2′-deoxycytidine (dC), 5-methyl 2′-deoxycytidine (mdC), 2′-deoxyadenosine (dA), 2′-deoxythymidine (dT), and 2′-deoxyguanosine (dG)—were completely separated by high-performance capillary electrophoresis in 10min. Intraday coefficient of variation was less than 1%, and interday coefficient of variation was less than 2%. The minimal detection limit was 1μM. Acquired drug resistance to methotrexate (MTX) is one of the most serious problems in cancer chemotherapy. Under optimal conditions, we analyzed global DNA methylation levels in A549 and A549/MTX cells, and only 105 cells are needed to obtain reliable results. The percentage of 5-methyl-2′-deoxycytidine (5-mC) was 4.80±0.52% in A549 cells, and this decreased to 4.20±0.44% in A549/MTX cells. It was considered as statistically significant. This demonstrated that the mechanisms of acquired drug resistance to MTX might be concerned with DNA methylation. [Copyright &y& Elsevier]