In the chronical experiment of treating with sulfur dioxide(SO2) inhalation, Micronuclei(MN) frequencies in the polychromatophilic erythroblasts(PCE) of mouse bone marrow and the frequencies of cells with MN were significantly increased in dose-dependent manner. There is a significant difference between the male and the female animals. The results also showed that SO2 inhibited urethone-induced MN formation, it is a antagonistic joint action to Urethone. These results furtherly confirm that SO2 inhalation is a clastogenetic and genotoxic agent to mammalian cells, and the combination roles of SO2 and other mutagens are complexity.