A specific inhibitor of DNA cytosine C-5-methylases would be useful for studying genomic imprinting, X-chromosome inactivation, carcinogenesis, and regulation of tissue-specific gene expression, for these physiological phenomena appears to be regulated through DNA methylation in promoter sequences. This paper reports a novel convenient in vitro assay method for screening DNA cytosine C-5-Methylase inhibitor. Our method uses a commercially available Hae III methylase (cytosine C-5 methylase), its corresponding Hae III endonuclease, and lambda DNA as their substrate.