We investigated an effect of polychlorinated dibenzofuran (PCDF) on pUC18 plasmid DNA. The mixtures of pUC18 and PCDF were introduced into the competent cells derived from E. coli. The competent cells were cultured on Luria-Bertani agar plate containing ampicillin. The number of colonies was not decreased in compared with it of E. coli inserted non-treated pUC18. The pUC18 re-extracted from the E. coli with PCDF-treated pUC18 were digested with 15 kinds of restriction enzymes and agarose gell electrophoresed. As the results no changes were found in compared with non-treated pUC18. Those findings suggested that the PCDF did not react with pUC18.