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Objective: The current study was designed to investigate the relationship between the patt of methylation of calcitonin (CT) gene and the activity of DNA methyltranferase (MTase). Methods: The methylation rate of the 5’ region of the CT gene (CTMR) and the activity of MTase in 6 solid tumor cell lines and 2 leukemia cell lines were determined. CTMR was detected by polymerase chain reaction (PCR) with inter and extal references in combination with restriction endonuclease and laser scan technique. MTase activity was examined by isotope labeled microassay. Results: Both CTMR and MTase activity in all tumor cell lines were significantly higher than that of control cells. The increased MTase activity was relative to elevated CTMR. Conclusion: There is prevalence of hypermethylation of CT gene and elevated activity of MTase in malignant cells. The increased MTase activity is one of the possible reasons for CT gene hypermethylation in tumor cells.