We isolated and characterized heparan sulfate rich proteoglycan (HSPG) present in the basement membrane from 7 M urea insoluble materials of the human placenta by CsCl isopycnic centrifugation, anion-exchange chromatography and gel filtration. The molecular mass of the glycosaminoglycan chain from pronase digestion of HSPG was estimated to be 30-100 kDa. The reported glycosaminoglycan moiety of proteoglycans from the lens capsule, glomerular basement membrane and fibroblast consists only of the heparan sulfate chain. However, the glycosaminoglycan of the proteoglycan isolated from human placenta was composed of heparan sulfate (82%) and other glycosaminoglycans (18%). An affinity-purified rat antibody against HSPG specifically reacted with HSPG, but it did not immunoreact with basement membrane components such as laminin and type IV collagen. Indirect immunohistochemical observations of tissue HSPG revealed staining mostly along with basement membrane (basal layer). From these results, isolated HSPG from human placenta appeared to be a constituent of the basement membrane, together with other basement membrane components such as type IV collagen, laminin and nidogen.