To study the inhibitory effect of RNA interference (RNAi) on MMP-9 gene expression in THP-1 cell line.To investigate the application of RNAi on the therapy of leukemia.Methods:Small interfering RNA ( siRNA) for MMP-9 gene was designed and transfected into THP-1 cells.MMP-9 mRNA expression was assessed by RT-PCR, and MMP-9 protein expression was tested by Western blot.MTT and trypan blue staining were used to observe the effect on the proliferation of THP-1 cells after RNAi.The changes in cell morphology were observed under the microscope.Results:The expressions of MMP-9 mRNA and protein were inhibited in THP-1 MMP-9 siRNA-transfected cells ,significantly lower than those of control cells.The results of MTT and trypan blue staining in-dicated that the proliferation ability of THP-1 cells obviously decreased after siRNA-transfected 48h and 72h.The growth of cells was in-hibited and the cells survival rate was significantly lower than that of control group ( P<0.05 ).The cells of control groups grew semi-quote wall under inverted microscope.The outline of cells was clear and the shape was uniform.The cells grew vigorously.While the growth of cells in siRNA group was inhibited.The morphology of siRNA group cells changed obviously by the Wright staining.Most cells expressed changes of apoptosis.Conclusion: siRNA for MMP-9 gene can not only reduce the expressions of MMP-9 mRNA and protein,but also inhibit the proliferation and induce apoptosis of THP-1 cells.