Studies were performed to determine the DNA interactions of and the induction of cytotoxic effects by the radical cation (CPZ+·) formed enzymatically from chlorpromazine (CPZ): in the presence of native DNA the lifetime of CPZ+· is markedly increased. The decreased reactivity of CPZ+· in the presence of native DNA and the concomitant increased viscosity of CPZ+·-DNA complexes strongly support the assumption that CPZ+· does form intercalation complexes with DNA. The relative strong bacteriotoxicity of CPZ+· hindered the accurate determination of mutagenesis in various Salmonella indicator strains, but a test for repairable DNA damage in Escherichia coli using various repair-deficient strains indicated that the cytotoxic action of CPZ+· is in part due to DNA alterations which can be excised in wild-type DNA repair-proficient strains. After activation of CPZ with long wavelength UV light, genetic effects are observed in S. typhimurium strain TA98, as well as in the E. coli tester strains. The possible role of CPZ+· in the photosensitization of CPZ is discussed.