Membrane-type 1 matrix metalloproteinase (MT1-MMP/MMP-14) has been believed a key enzyme in tumor invasion, because it is expressed in a variety of malignant human tumors, and overexpression of the enzyme enhances the ability of cellular invasiveness. However, it has not necessarily been clarified whether the endogenously expressed MT1-MMP in human tumors plays a critical role in their invasiveness. We used RNA silencing technology to downregulate the endogenous MT1-MMP expression in human tumor cells (fibrosarcoma HT1080 and gastric carcinoma MKN-28 cell lines), and evaluated the effect on the invasion of a reconstituted basement membrane (Matrigel). Transfection of a double-stranded RNA targeted to the MT1-MMP gene decreased the level of the enzyme to less than 10-20% without affecting production of other MMPs. According to the degree of silencing, activation of proMMP-2 was inhibited. CD44 shedding was also inhibited, but only in part. Decreased MT1-MMP levels were also reflected in reduced cell motility on hyaluronan (HA) and invasion in Matrigel. Thus, specific downregulation of MT1-MMP expression was sufficient to cause significant inhibition of the migration and invasion of tumor cells, even though other MMPs continued to be expressed.