Nitrogen mustard and nor-nitrogen mustard were investigated in Salmonella typhimurium and human lymphocytes for genotoxicity. Point mutations were assessed using the plate test, the conventional spot test and a method in which the substances were not in direct contact with the microorganisms. Both compounds were active, but nor-nitrogen mustard was far more potent than nitrogen mustard in all bacterial systems. Cytogenetic experiments with human lymphocytes revealed that both compounds induced a dose dependent increase in chromosomal aberrations and sister chromatid exchanges, but that nitrogen mustard was 10 times more potent than nor-nitrogen mustard. Thus, the spectrum of activity for nor-nitrogen mustard and nitrogen mustard differ. Nor-nitrogen mustard was more effective in inducing point mutation damage than nitrogen mustard and a reversal of potency was found for cytogenetic damage. These results indicate that although these two substances induce the same type of DNA lesions the amounts of the different DNA adducts vary.