In a sense, cancer chemotherapy is similar to antibacterial chemotherapy. The main target for both chemotherapies is on DNA and its precursors. Whenever anticancer drugs exhibit its activity on cancer cells, damage or metabolic abnormality of DNA is usually accompanied. Among various anticancer drugs, the main cellular target of alkylating agents or anticancer antibiotics is DNA strand, and many of newly introduced anticancer drugs exert effect on DNA strand or DNA synthesis. The mechanism of action of quinocarmycin, KT 6149, which is a derivative of mitomycin C, YM534, which shows thrombocyte agglutination activity, and MCNU, a nitrosourea, were studied for their antitumor action and damage to DNA strand. It was found that there was a close relationship between anticancer effect and single strand scission of DNA in all the agents tested.