OBJECTIVE To study the action mechanism of a Chinese herbal mixture PC-SPES II inducing the apoptosis of androgen independent prostate adenocarcinoma cell line (PC-3). METHODS The growth of PC-3 was shown by MTT. Immunofluorescence staining of acridine orange (AO) and flow cytometry were used to detect the apoptosis. The expressions of the apoptosis-related proteins were analyzed with their monoclonal or polyclonal antibodies after Western blotting. RESULTS PC-SPES II not only inhibited the growth of PC-3 cells but also induced their death. The apoptosis of PC-3 cells treated with PC-SPES II was detected by immunofluorescence staining of AO and flow cytometry, which showed the apoptotic cells to be (29.8 +/- 5.6)%, but the untreated control cells (0.06 +/- 0.014)%, (P < 0.01). The expression of Bcl-2 and Bcl-xL, two antiapoptosis proteins, was decreased while Bax, a pro apoptosis protein, was elevated in the cells treated with PC-SPES II as compared with the untreated control (P < 0.01). Accordingly, the expression of the activated fragments of caspase-3 was also increased (P < 0.01). CONCLUSIONS The Chinese herbal mixture PCSPES II can induce the apoptosis of PC-3. Its mechanism may lie in the up-regulation of Bax expression and down-regulation of Bcl-2 and Bcl-xL expressions, which induce the activated fragments of caspase-3 by mitochondria.