In this study, the genotoxic effects of gemcitabine and topotecan were investigated in mouse bone marrow cells using the micronucleus and chromosomal aberration test systems. Gemcitabine increased the frequency of micronuclei, particularly at the median dose for the 24-, 36-, and 48-h sampling intervals. It had cytotoxic effects on the bone marrow and decreased the polychromatic/normochromatic erythrocyte ratio dose-dependently for all sampling intervals. Gemcitabine significantly decreased the mitotic index at the 24-h time point. It increased the number of abnormal cells and induced a significant increase in total chromosomal aberrations. For the 6-h sampling time, gemcitabine neither induced chromosomal aberrations nor reduced the mitotic index. Topotecan also induced high levels of micronuclei, particularly for the 24- and 36-h sampling times and it decreased the polychromatic/normochromatic erythrocyte ratio for all sampling intervals, which is indicative of bone marrow cytotoxicity. The bone marrow metaphase analysis showed that topotecan significantly elevated the number of abnormal metaphases and total chromosomal aberrations at 6 and 24h, in a dose-dependent manner. It also decreased the mitotic index for both sampling intervals. In conclusion, the results of this study indicate that the two chemotherapeutics gemcitabine and topotecan have cytotoxic and genotoxic effects in mouse bone marrow. (C) 2003 Elsevier Science B.V. All rights reserved.