We have isolated a unique, basement membrane proteoglycan from the Engelbreth-Holm-Swarm (EHS) sarcoma. This proteoglycan, estimated to be 0.75 X 10(6) daltons, was found to contain about equal amounts of protein and covalently linked heparan sulfate. Antibody prepared against this proteoglycan reacts with the basement membrane matrix in the tumor and with the basement membranes in skin, kidney, and cornea. These studies indicate that the heparan sulfate proteoglycan is a normal constituent of basement membranes that presumably plays an important role in the organization of basement membrane components and that also may determine the permeability of basement membranes to acidic molecules.