The frequencies of sister-chromatid exchange (SCE) in peripheral blood lymphocytes of 40 workers at a phosphate fertilizer factory in North China were studied. HF and SiF4 are main air pollutants in the factory, there is also some dust containing fluoride, phosphate fog, NH3 and SO2. It was shown that the chemicals caused an increase in SCE, and also induced cell mitotic delays. The mean SCEs/cell of the workers and the non-exposed controls were 7.47 +/- 0.31 and 4.94 +/- 0.14 (p < 0.01) respectively. SCEs/cell in 75% of 40 workers were higher than 6 while 40 controls all had values lower than 6. SCE frequencies of the workers increased with length of the chemical exposure period up to 10 years. Smoking enhanced the SCE frequencies induced by the chemicals.