Data from four mouse bone marrow micronucleus assays are presented in which sister slides were stained with Giemsa or Acridine Orange (AO). These data are used to support our decision to change from Giemsa stain to AO when conducting rodent bone marrow micronucleus assays. The AO method leads to definitive assessments of the incidence of micronucleated polychromatic erythrocytes on a slide, and in so doing, it saves reading time and increases confidence in data. Preliminary experiments with DMBA using the triple-dose/single sample protocol of MacGregor are also described, and it is concluded that this protocol shows great promise.