Endothelial, epithelial and muscle cells produce a similar proteoglycan for deposition in basement membrane. This proteoglycan is initially synthesized as a low buoyant density proteoglycan containing 3-5 heparan sulphate side-chains (15,000-65,000 Mr each), along one half of a 400,000 Mr core protein. A portion of the population of these macromolecules is degraded to produce small high density proteoglycans containing a variable-sized core protein less than 100,000 in Mr. This biosynthetic and degradative process probably accounts for the variety of differently sized heparan sulphate proteoglycans that have been isolated from various basement membrane sources.