Bacterial redox protein azurin induce apoptosis in human osteosarcoma U2OS cells. As a low molecular weight redox protein elaborated from the pathogenic bacteria Pseudomonas aeruginosa, azurin is one of representative bacterial products applied in the treatment of tumour. We found that the growth of U2OS cells was significantly inhibited by azurin in a dose-dependent manner with the IC(50) value of 114.54+/-7.65 mgl(-1). But the growth of MG63 cells or L02 cells was almost not inhibited by azurin (P<0.05). Moreover, when treated with azurin, U2OS cells showed typical apoptotic morphological features observed by fluorescent microscopy (AO and Hoechst 33258) and transmission electron microscopy. Typical DNA \ladder\ bands were also observed. The apoptosis rate was 35.8% tested by fluorescence-activated cell sorter (Annexin-V-FITC(+)/PI(-)) and the cell-cycle arrested in G(1) phase. But no apoptotic features were observed in control cells. The down-regulation of Bcl-2 (an inhibitor of apoptosis) were detected in U2OS cells when azurin was added for 24h. In contrast, the level of Bax and caspase-3 were significantly up-regulated. So we concluded that azurin could selectively induce apoptosis of human osteosarcoma U2OS cells and the induction of apoptosis by azurin was closely associated with down-regulation of Bcl-2, up-regulation of Bax and activation of caspase-3.