1,2-Dimethylhydrazine showed dose-related clastogenic activity in the mouse bone marrow micronucleus assay following intubation of an aqueous solution of the dihydrocholoride salt (10 and 50 mg/kg, 24 h sampling). These data counter earlier reports of the inactivity of this material in a rat and a mouse micronucleus assay, following its intraperitoneal injection. In the present study, 2,000 polychromatic erythrocytes were assessed per animal. However, had only 300 cells been assessed for micronucleated polychromatic erythrocytes, it is shown that a negative result could have been recorded. This may explain the earlier and negative results.