The activity of MNNG in the mouse bone marrow micronucleus assay

N-Methyl-N'-nitro-N-nitrosoguanidine (MNNG) is established herein as reproducibly active in the mouse bone marrow micronucleus assay. Activity was evident in two strains of mice, using suspensions in water or corn oil, and using two routes of exposure (oral gavage and i.p. injection). Activity was greatest when using the i.p. injection route, but a weak positive response was evident following oral gavage dosing of MNNG as an homogenate in corn oil. The latter weak response was statistically significant and was confirmed by both extended assessment of the slides and by re-analysis of selected slides in an independent laboratory. However, only some of the animals responded, as observed by others earlier. It is concluded that several earlier literature reports of the inactivity of this agent in this assay must be due either to the hydrolysis of MNNG prior to dosing, or to inadequate methods of data acquisition.