The authors investigated the micronucleus frequencies (MP) in peripheral lymphocytes in 584 workers exposed to five kinds of soft wood dust and found that all kinds of soft wood dust could increase the micronucleus frequency. As the control's 95% confidential upper limit > or = 0.4% is defined as the standard for positivity the micronucleus positive frequency (MPF) in the exposure group is much higher than that in the control group (P < 0.01). Meanwhile the increase of exposure years may increase the MP, and the time-response relationship is evident (r = 0.90, P < 0.01). For different exposures, the MPFs are: 36.2% for linden, 26.3% for fir. 15.7% for lacebark pine, 9.6% for birch and 2.1% for polar. The positive rate of linden and fir exposure are significantly higher than that of the control group (P < 0.01). Water extract of these five kinds of soft wood dust can increase the frequencies of micronucleus in polychromatic red blood cell in mice's sternum marrow, which also showed a dose-response. The correlation coefficients are: 0.86 for linden 0.6 for fir, 0.39 for lacebark pine, 0.76 for birch and 0.73 for polar. All but the lacebark pine have the P value less than 0.01 (P < 0.01). At the dose of 15 g/kg body weight, micronucleus frequency induced by linden was the highest (0.8%), next were fir (0.71%), birch (0.65%), lacebark pine (0.63%), and polar (0.43%). The results from animal test are consistent with those from the exposure workers. These results suggest that there are some substances in soft wood which can induce chromosome aberration.