The irreversible binding of chlorpromazine radical cation (CPZ+.) and photoactivated chlorpromazine (CPZ) to calf thymus DNA in vitro and bacterial macromolecules in intact bacterium cells was investigated. CPZ+. may be formed in vivo metabolically and photochemically. CPZ+. and photo-activated CPZ bind covalently to double- and single-strand DNA. The conformation of the DNA appeared to be important for the CPZ+. reactivity: though CPZ+. is less stabilized by complex formation with single-strand DNA, the reaction rate and the binding capacity of DNA-complexed CPZ+. with single-strand DNA is larger than with double-strand DNA. Photoactivated CPZ binds considerably to proteins, DNA and RNA in the intact bacterium cells. In spite of the relatively short lifetime of CPZ+. in the presence of the cells CPZ+. also binds irreversibly to bacterial DNA, RNA and proteins. The consequences of covalent binding for the cytotoxicity and genotoxicity of CPZ+. and photoactivated CPZ and the possible role for CPZ+. as an intermediate in the photobinding of CPZ is discussed.