Gelatinase A is secreted as a proenzyme (progelatinase A) which is activated and bound on the surface of tumor and normal cells. We have reported that the expression of a membrane-type-1-matrix metalloproteinase (MT1-MMP) induces activation of progelatinase A. Here we demonstrate that the expression of MT1-MMP in COS-1 cells induces cell-surface binding of progelatinase A which is consequently processed to an intermediate form. Processing from the intermediate to the fully active form is dependent on the gelatinase A concentration. These results suggest that the cell-surface binding concentrates the gelatinase A intermediate form locally to allow autoproteolytic processing to the fully active form.