Contact activation occurs when plasma comes in contact with negatively charged man‐made surfaces but no substance that initiates contact activation in vivo has been identified. We have isolated a mast cell heparin proteoglycan (MC‐HepPG) from a Furth mouse mastocytoma‐derived cell line that is analogous to human tissue‐type mast cell HepPG. This material and other glycosaminoglycans (GAGs) were tested for their ability to accelerate the reciprocal activation of factor XII and prekallikrein and the autoactivation of factor XII. Quantitative analysis showed the MC‐HepPG to be as active as dextran sulfate on a weight basis; hog intestine heparin, dermatan sulfate. keratan polysulfate and chondroitin sulfate C were less active, other sulfated polysaccharides were essentially inactive. Incubation of MC‐HepPG in 1:4 diluted plasma resulted in complete cleavage of high molecular weight kininogen in a factor Xll‐dependent reaction. All of the MC‐HepPG dependent reactions described above were inhibited by preincubation of MC‐HepPG with heparinase I and II but not by pretreatment with heparitinase. chondroitinase ABC or the serine protease inhibitor aPMSF thus indicating that heparin proteoglycan is indeed acting as an initiating ‘surface’. We analysed the proteoglycan preparation by HPLC gel filtration. Fractions spanning a molecular weight range of > 400000–8000 were active initiators. Comparison of the chromatograms obtained before and after cleavage of GAG side chains from the protein core suggested that dissociated GAGs in the MW range 69000–17000 are the most active species rather than the complete proteoglycan. MC‐HepPG GAGs therefore represent a physiologic macromolecule with activity comparable to non‐physiological surfaces in a purified system and with the capability to induce activation of the contact system in diluted plasma. Its ability to promote kinin generation links cellular and humoral inflammatory responses in the perivasculature and provides a possible explanation for the elevated kinin levels observed after allergen exposure.