Carcinogens: 7,12-dimethylbenz(a)anthracene (DMBA), 2-acetylaminofluorene (AAF), 3-methylcholanthrene (MC), daunorubicin, thio-TEPA, cyclophosphamide, 1-(2-chloroethyl)2-cyclohexyl-1-nitrosourea (CCNU) and mitomycin C, which mutate Salmonella typhimurium tester strains in the Salmonella/microsome mutagenicity test, strongly stimulate the in vitro synthesis of DNA isolated from His- mutants (TA-1538, TA-1537, TA-1535), and induce in vitro His- DNA strand separation. Four other carcinogens, ethionine, actinomycin D, bleomycin and reserpine, which were not known as mutagens in the Ames test, also strongly stimulate His- DNA synthesis and His- DNA strand separation in local areas. Those substances which are neither carcinogenic nor mutagenic: d-lactose, fluorene, cholesterol, and saccharin, do not stimulate either His- DNA (or His+ DNA) synthesis or DNA in vitro strand separation. Steroid hormones, which are carcinogenic only for steroid target tissues, do not react with bacterials DNA.