The effect of novobiocin (NOV), an inhibitor of DNA topoisomerase II, on the induction of chromosome aberrations and sister chromatid exchanges (SCEs) by ethyl methanesulfonate (EMS) or bleomycin (BLM) was investigated in synchronized Chinese hamster ovary (CHO) cells. EMS showed the sensitivity to the induction of chromosome aberrations and SCEs in late G₁ and early S phases of the cell cycle, whereas BLM showed the stage sensitivity to chromosome aberrations in G₂ phase. The stage sensitivity on SCEs induced by BLM was not shown in this experiment. NOV induced neither chromosome aberration nor SCEs by itself. Posttreatment with NOV after EMS or BLM did not affect the frequencies of chromosome aberrations and SCEs. When cells were pretreated with NOV before treatment with EMS plus NOV, increased chromosome aberrations were observed at late G₁ and early S phases.