In an attempt to shed light on the influence of tritiated water for DNA we have investi-gated the damage with a simple Plasmid DNA, pBR 322. The survival of covalently closed circular (CCC DNA) form was directly followed by agrose gel electrophoresis. It was found that the survival percentage of DNA in tritiated water was observed almost the same as the irradiation of X-rays at the same absorbed doses. For the irradiation of γ-rays, on the other hand, the decay rate was larger than those of both tritiated water and X-rays. The yields percentage of the broken pieces of DNA in tritiated water, X-rays and γ-rays were found to be 43, 38 and 33% at 104 rad of the absorbed dose. It may be considered that the degree of danger in tritiated water is quite larger than that of γ-rays. It was also found that the dose rate effect was not observed in the case of tritiated water, X-rays and 7-rays irradiation.