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 10 min. Intraday coefficient of variation was less than 1%, and interday coefficient of variation was less than 2%. The minimal detection limit was 1 microM. 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 10(5) 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.