Tumour cells traverse epithelial and endothelial basement membranes during the successive stages of the metastatic process. At the transition from in situ to invasive carcinoma, local dissolution of the basement membrane is observed microscopically1,2, and coincides with tumour cell invasion of the underlying stroma. Tumour cells further traverse the endothelial basement membrane during entry into and egress from blood vessels3-5. Electron microscopic studies have shown local dissolution of basement membrane at its area of contact with invading tumour cells, suggesting an enzymatic mechanism3,6,7. Basement membranes are resilient structures which present a mechanical barrier to invasion8. Type IV collagen is a major structural protein of basement membranes and is chemically and genetically distinct from stroma collagen types I and III and cartilage collagen type II9,10. Previously characterised animal collagenases which cleave collagen types I; II and III fail to degrade type IV collagen11,12. We have recently purified about 1,000-fold and characterised a neutral protease activity preferential for type IV collagen from metastatic tumour cells and shown that it (1) produces specific degradation products, (2) has a molecular weight of 65,000, (3) is not plasmin or a cathepsin, by pH and inhibitor studies, and (4) does not significantly degrade other collagens or fibronectin12,13. Here we extend the relevance of this finding by quantitating the ability of several murine tumour cell lines of known metastatic potential to degrade type IV collagen. The cell lines with the highest incidence of spontaneous metastasis exhibit the greatest level of type IV collagen-degrading activity in two different assays using either living cells or media obtained from cell cultures.