Heparan sulphate (HS) is an important component of the extracellular matrix (ECM) and the vasculature basal lamina (BL) which functions as a barrier to the extravasation of metastatic and inflammatory cells. Platelet-tumour cell aggregation at the capillary endothelium results in activation and degranulation of platelets. Cleavage of HS by endoglycosidase or heparanase activity produced in relatively large amounts by the platelets and the invading cells may assist in the disassembly of the ECM and BL, and thereby facilitate cell migration. Using a recently published rapid, quantitative assay for heparanase activity towards HS [Freeman, C. and Parish, C. R. (1997), Biochem. J., 325, 229-237], human platelet heparanase has now been purified 1700-fold to homogeneity in 19% yield by a five column procedure, which consists of concanavalin A-Sepharose, Zn²⁺-chelating-Sepharose, Blue A-agarose, octyl-agarose and gel filtration chromatography. The enzyme, which was shown to be an endoglucuronidase that degrades both heparin and HS, has a native molecular mass of 50 kDa when analysed by gel filtration chromatography and by SDS/PAGE. Platelet heparanase degraded porcine mucosal HS in a stepwise fashion from a number average molecular mass of 18.5 to 13, to 8 and finally to 4.5 kDa fragments as determined by gel filtration analysis. Bovine lung heparin was degraded from 8.9 to 4.8 kDa while porcine mucosal heparin was degraded from 8.1 kDa to 3.8 and finally to 2.9 kDa fragments. Studies of the enzyme's substrate specificity using modified heparin analogues showed that substrate cleavage required the presence of carboxyl groups, but O- and N-sulphation were not essential. Inhibition studies demonstrated an absolute requirement for the presence of O-sulphate groups. Platelet heparanase was inhibited by heparin analogues which also inhibited tumour heparanase, suggesting that sulphated polysaccharides which inhibit tumour metastasis may act to prevent both tumour cell and platelet heparanase degradation of endothelial cell surface HS and the basal laminar.