The mechanism of genotoxic potential of the cancer chemotherapeutic drugs amsacrine and nocodazole in mouse bone marrow was investigated using a micronucleus test complemented by fluorescence in situ hybridization assay with mouse centromeric and telomeric DNA probes. In animals treated with different doses of amsacrine (0.5–12 mg kg−1), the frequencies of micronucleated polychromatic erythrocytes increased significantly after treatment with 9 and 12 mg kg−1. A statistically significant increase in micronuclei frequency was also detected for 75 mg kg−1 nocodazole (two exposures, spaced 24 h apart). Both compounds caused significant suppressions of erythroblast proliferation at higher doses. Furthermore, the present study demonstrated for the first time that amsacrine has high incidences of clastogenicity and low incidences of aneugenicity whereas nocodazole has high incidences of aneugenicity and low incidences of clastogenicity during mitotic phases in vivo. The assay also showed that chromosomes can be enclosed in the micronuclei before and after centromere separation. Therefore, the clinical use of these genotoxic drugs must be weighed against the risks of the development of chromosomal aberrations in cancer patients and medical personnel exposed to drug regimens that include these chemicals. Copyright © 2011 John Wiley & Sons, Ltd.