Cancer cells are known to shed extracellular membrane vesicles both in vitro and in vivo. To analyse their possible involvement in the metastatic behaviour of tumours, we measured the Matrigel invasion capability and amounts of vesicles shed by four human tumour cell lines (8701-BC, MCF-7, MDA-MB-231 and HT-1080), and by MCF-10A, an immortalised human breast cell line. The proteolytic activity content of vesicles was analysed by gelatin and casein zymographies. While MCF-10A cells do not release a measurable amount of vesicles, all tumour lines analysed, when cultured in presence of serum, shed vesicles rich in MMP-9. Other vesicle-associated proteinases include MMP-2 and uPA. Amounts and proteolytic activities of shed vesicles correlate with the in vitro invasiveness of cells. Since vesicles appear to promote the proteolytic cascade required for the localised degradation of the extracellular matrix, their shedding from cancer cells might represent an important feature of tumour progression.