Frequencies of lymphocytes with micronuclei (MNF) in human peripheral blood cultures from workers in a sulphuric acid factory (the SO2 concentrations in the workplace ranged from 0.34 mg/m3 to 11.97 mg/m3 air) were investigated. It was shown that among 40 workers, the MNF of 28 workers was more than 0.1%, and the MNF of 7 workers was greater than the upper limit of the normal range (0.2%). The mean MNF of the worker and the control group was 0.168% and 0.071%, respectively. The difference between them was statistically significant (P less than 0.001). It implies that SO2 is a clastogenic and genotoxic agent. No positive correlation between the MNF and length of service in workers has been observed. In the control population, the MNF of smokers was significantly higher than that of non-smokers (P less than 0.001).