Fine particulate matters (PM_2.5) collected in Beijing during a period from September 2005 to May 2006 were studied.Sequential extraction procedures were applied to divide the total lead into three fractions,i.e.water soluble,liposoluble and insoluble lead.Lead concentrations and their isotopes in each fraction were then determined by inductively coupled plasma mass spectrometry (ICP-MS).Lead standard reference GBW 09133 was used to investigate the accuracy of lead concentration determination and SRM 981 was used to correct the mass discrimination and instrumental drift.The obtained results showed that the analytical precision of lead isotope ratios for SRM 981 of seven repeat measurements at lead concentration of 10 ng·mL^-1 came to about 0.34,0.27 and 0.24 percent for the 206Pb/204Pb,206Pb/207Pb and 206Pb/208Pb ratios,respectively.In Beijing,the mass concentrations for PM_2.5 and lead of ambient air were 125.56 and 0.415 μg·m-3 respectively for winter 2005,and 201.6 and 0.475 μg·m-3 respectively for spring 2006 on average,resembling those published results.It was indicated that the lead in the PM_2.5 existed mainly in insoluble form,which accounted for 78.99%,while water soluble lead and liposoluble lead amount were 20.69% and 0.32%,respectively.The mean values of 206Pb/207Pb ratio for water soluble species and insoluble species were 1.152 6±0.009 3 and 1.219 3±0.009 1 respectively,with a significant difference (Student’s test;p＜0.01) statically.By comparing 206Pb/207Pb ratios between biological specimen and ambient air samples,the 206Pb/207Pb ratios in water soluble fraction coincided very well with those in adult blood,therefore,water soluble lead but not insoluble lead in PM_2.5 may be the potential source of adults blood lead in Beijing.More attention should be paid to the effect of soluble lead in fine particle matters on human health.