Bupleuri radix, a traditional Chinese herb, has been widely used to treat liver diseases such as hepatitis and cirrhosis. The acetone extract of Bupleurum scorzonerifolium (AE-BS) showed a dose-dependently antiproliferative effect on the proliferation of A549 human lung cancer cells. The IC(50) of AE-BS, i.e., the concentration required to inhibit proliferation of A549 cells, was 59 +/- 4.5 microg/ml on day 1. The IC(50) of AE-BS for WI38 human normal lung fibroblast cells, however, was significant higher than that for A549 cells (150 +/- 16 microg/ml, p< 0.01). After 72 hours of exposure, AE-BS (60 microg/ml) significantly reduced A549 cell proliferation to 33 +/- 3.2% of control. In TUNEL assay, A549 cells treated with AE-BS showed typical morphologic features of apoptosis, and the percentage of apoptotic cells was approximately 38 % on day 1. In the TRAP assay, AE-BS-treated cells demonstrated significantly lower telomerase activity on day 3. This result indicates that the AE-BS could suppress the proliferation of lung cancer cells via inhibition of telomerase activity and activation of apoptosis.