The frequencies of chromosomal aberrations and sister-chromatid exchanges (SCE) in peripheral blood lymphocytes of 40 workers chronically exposed to sulphur dioxide (SO2) at a sulphuric acid factory in Taiyuan City (North China), were studied. It was shown that the mean frequency of chromosomal aberrations and the mean frequency of lymphocytes with chromosomal aberrations of the SO2-exposed workers were both higher than the controls. The mean per 1000 metaphase frequencies of severe chromosomal aberration types (chromosome rings, translocations, and dicentrics) of the workers and the controls were 9.63 and 2.27, respectively. The difference between them was statistically significant (p less than 0.01). It was also shown that the mean SCEs/cell of SO2-exposed workers and non-exposed controls were 6.72 +/- 0.22/cell and 2.71 +/- 0.13/cell (p less than 0.01) respectively. SCEs/cell in 39 workers were all higher than 5, only 1 worker was 4.73. However, 41 controls were all lower than 4, only 1 control was 4.92. The difference between the worker and the control group was statistically significant. These results show that SO2 is a clastogenic and genotoxic agent. No positive correlation between the frequencies of chromosomal aberrations or SCE and length of service in the workers has been observed. No significant difference between smokers and non-smokers was found in these assays.