[en] Membrane-type I matrix metalloprotemase (MT1-MMP) is a key enzyme in normal development and malignant processes. The regulation of MT1-MMP activity on the cell surface is a complex process involving autocatalytic processing, tissue inhibitor of MMPs (TIMP) binding and constitutive internalization. However, the fate of internalized MT1-MMP is not known. Acidification of intracellular vacuolar compartments is essential for membrane trafficking, protein sorting and degradation. This acidification is controlled by vacuolar H+-ATPases, which can be selectively inhibited by bafilomycin-A(1). Here, we treated human tumour cell lines expressing MT1-MMP with bafilomycin-A(1), and analysed its effects on MT1-MMP activity, internalization and processing. We show that the activity of MT1-MMP on the cell surface is constitutively down-regulated through a vacuolar HI-ATPase-dependent degradation process. Blockade of this degradation caused the accumulation of TIMP-free active MT1-MMP molecules on the cell surface, although internalization was not affected. As a consequence of this impaired degradation, pro-MMP-2 activation was strongly enhanced. This study demonstrates that the catalytic activity of MT1-MMP on the cell surface is regulated through a vacuolar H+-ATPase-dependent degradation process.