The precursor protein of a basement membrane specific heparan sulfate proteoglycan has been identified as a 400,000 Mr polypeptide. Antibodies against large and small forms of this proteoglycan, isolated from a basement membrane (Engelbreth-Holm-Swarm, EHS) tumor, immunoprecipitated the same 400,000 protein from pulse-labeled EHS cells. The proteoglycan precursor protein was not recognized by antibodies against other basement membrane components or by antibodies to the cartilage proteoglycan. Furthermore, heparan sulfate proteoglycan purified from the EHS tumor blocked the immunoprecipitation of the precursor protein. Pulse-chase studies with [35S]methionine showed the precursor protein was converted to a proteoglycan. Pulse-chase studies with 35SO4 showed the large, low density proteoglycan appeared first and was degraded to a smaller, high density proteoglycan. We propose that the precursor protein is used after very little or no modification in the assembly of a large, low density heparan sulfate proteoglycan and that a portion of the population of these macromolecules are subsequently degraded to a smaller form.