OBJECTIVE To explore the effects of 12-O-tetradecanoylphorbol-13-acetate (TPA) on differentiation, apoptosis and related molecular mechanisms in U937 myelomonocytic leukemia cells.METHODS Morphological changes were analyzed by phase contrast and light microscopy, expression of the monocytic differentiation maker CD11b by direct immunofluorescence staining, cell cycle distribution and apoptosis by flow cytometry, and expression of bcl-2, Bax, survivin and p21Cip1/Waf1proteins by West analysis.RESULTS Treatment of U937 cells with 10 nmol/L TPA induced cell adherence. The adherent cells showed G0/G1 cell cycle arrest (69.0% at 24h vs 52.1% control; P< 0.01),and morphologic changes and increased expression of the monocytic differentiation marker CD11b (63.0% at 72 h vs15.3% control; P< 0.01 ). In addition to these effects, about 20% of the cells still remained in suspension and exhibited a time-dependent increasing apoptosis, which reached 70.3% after 72 h of treatment ( P< 0.01 ). TPA treatment for 24 h induced expression of p21Cip1/Waf1 in the adherent cells, but not in the non-adherent cells. Furthermore, bcl-2 and survivin expression declined in 24 h-TPA-treated non-adherent cells compared with untreated control and adherent cells, whereas no change in the expression of Bax was detected.CONCLUSION TPA induces both differentiation and apoptosis in U937 cells,which may be related to the upregulation of p21Cip1/Waf1 and downregulation of bcl-2 and survivin expression.