We previously reported a high concordance of in vitro micronucleus (MNvit) results obtained by flow cytometry to the known cytogenetic activity often commercially available compounds mentioned as validation compounds in an early draft of the OECD MNvit TG487 [Bryce et al., 2010; Organization for Economic Co-operation and Development(OECD), 2007]. The current study investigated this method in Chinese hamster V79 cells with pharmaceutical compounds of unknown genotoxic potential. Twenty-five compounds from several therapeutic areas such as oncology, neuroscience and immunological research were tested in the flow cytometry assay, and for comparison using the cytokinesis-block microscopy assay. Five of these 25 compounds were considered positive for micronucleus induction by the microscopy assessment. In all cases, the results from the flow cytometry assess ment matched the results of the microscopy assay. Thus, flow cytometry is a viable method for assessing the aneugenic/clastogenic potential of pharmaceutical drug candidates. The flow method offered several advantages over traditional microscopy. For instance, the ratio of micronuclei (MN) to 10,000 nuclei was evaluated in less than 2 min vs.15 min to manually assess 600 binucleate cells. Evaluation by flow cytometry can be automated,freeing resources and eliminating scorer fatigue.The assay may also provide for mechanistic understanding of MN formation based on size and the ratio of nuclei with sub-2N DNA content, allowing for discrimination between aneugenic and clastogenic compounds.