The mutagenicity of six heterocylic nitrogen mustards (ICR compounds) has been determined in a cultured mammalian cell system by use of resistance to the purine analog 6-thioguanine to select for mutation induction at the hypoxanthine-guanine phosphoribosyltransferase locus in Chinese hamster ovary cells. The six compounds tested are ICR 191, 170, 292, 372, 191-OH, and 170-OH. The first four contain a single 2-chloroethyl group (nitrogen half-mustard) on the side chain and are mutagenic, with the tertiary amine types (170 and 292) 3 to 5 times more mutagenic than the secondary amine types (191 and 372). The remaining two compounds (191-OH and 170-OH) are not mutagenic, indicating that the 2-chloroethyl group is needed for mutation induction.