We investigated the reaction of DNA with pyrocatechol under the assumption that chromosomal abnormalities concern with cancer phenomena. Pyrocatechol enhances its antitumor potency under co-existence with Cu^2+ in spite of decreased administration. This result suggests that oxidative process and oxidation product of pyrocatechol have some significance to its antitumor function. Mixtures of pyrocatechol and Cu-salts remarkably lowered the viscosity of DNA solution at neutral pH. That DNA treated by this mixture still maintains double stranded nature is evident from analyses of Tm curves and insensitivity to formaldehyde. Experiments further showed that the oxidation product which is formed from catechol in the presence of Cu combines specifically with apurinic acid, but does not react with apyrimidinic acid or pyrimidin mononucleotides. So it is supposed that a certain chain-length of pyrimidinoligonucleotides are indispensable for such reactions. It is now assumed that the pyrocatechol-Cu^2+ F breakes some portions of pyrimidin clusters in nuclear DNA, killing thus cancer cells.