The antipsychotic drug chlorpromazine causes scission of the DNA in PY815 mouse mastocytoma cells or isolated PY815 cell nuclei and the broken DNA reseals when chlorpromazine is removed from nuclei. These properties suggest that chlorpromazine interferes with topoisomerase action as do several other DNA-intercalating anti-cancer drugs. However, protein is not associated with the broken DNA after chlorpromazine treatment suggesting a different mode of action on the topoisomerase. Reasons why chlorpromazine may have potential as anti-cancer agent are considered.