Two topoisomerase II inhibitors, etoposide and merbarone, were tested for the induction of dominant lethal mutations in male mice. Etoposide was administered at a dosage of 30 or 60 mg/kg. Merbarone was administered at a dosage of 40 or 80 mg/kg. These males were mated at weekly intervals to virgin females for 6 weeks. In the present experiments, regardless of the agent, spermatids appeared to be the most sensitive germ-cell stage to dominant lethal induction. Etoposide and merbarone clearly induced dominant lethal mutations in the early spermatid stage only with the highest tested doses. The mutagenic effects were also directly correlated with reactive oxygen species accumulation as an obvious increase in 2',7'-dichlorofluorescein fluorescence level was noted in the sperm of animals treated with higher doses of etoposide and merbarone. Treatment of male mice with N-acetylcysteine significantly protected mice from etoposide- and merbarone-induced dominant lethality. Moreover, N-acetylcysteine treatment had no antagonizing effect on etoposide- and merbarone-induced topoisomerase II inhibition. Overall, this study provides for the first time that etoposide and merbarone induce dominant lethal mutations in the early spermatid stage through a mechanism that involves increases in oxidative stress. The demonstrated mutagenicity profile of etoposide and merbarone may support further development of effective chemotherapy with less mutagenicity.