The successful penetration of endothelial basement membranes is an important process in the formation of hematogenous tumor metastases. Heparan sulfate (HS) proteoglycan is a major constituent of endothelial basement membranes, and we have found that HS‐degradative activities of metastatic B16 melanoma sublines correlate with their lung‐colonizing potentials. The melanoma HS‐degrading enzyme is a unique endo‐β‐D‐glucuronidase (heparanase) that cleaves HS at specific intrachain sites and is detectable in a variety of cultured human malignant melanomas. The treatment of B16 melanoma cells with heparanase inhibitors that have few other biological activities, such as N‐acetylated N‐desulfated heparin, results in significant reductions in the numbers of experimental lung metastases in syngeneic mice, indicating that heparanase plays an important role in melanoma metastasis. HS‐degrading endoglycosidases are not tumor‐specific and have been found in several normal tissues and cells. There are at least three types of endo‐β‐D‐glucuronidases based on their substrate specificities. Melanoma heparanase, an M_r ∼96,000 enzyme with specificity for β‐D‐glucuronosyl‐N‐acetylglucosaminyl linkages in HS, is different from platelet and mastocytoma endoglucuronidases. Elevated levels of heparanase have been detected in sera from metastatic tumor‐bearing animals and malignant melanoma patients, and a correlation exists between serum heparanase activity and extent of metastases. The results suggest that heparanase is potentially a useful marker for tumor metastasis.