The primary structure of murine betaglycan, also known as transforming growth factor beta (TGF-beta) type III receptor, was deduced from the nucleotide sequence of a cDNA clone isolated from a heart library. Murine betaglycan is a single spanning membrane polypeptide of 850 amino acids which is highly similar to betaglycan of other species. Transfection of this cDNA into COS1 cells resulted in the expression of a membrane proteoglycan that binds TGF-beta and is recognized by antibodies raised against rat betaglycan. COS1 cells transfected with the double mutant Ser533Ala; Ser544Ala of the murine betaglycan cDNA produced a TGF-beta type III receptor devoid of glycosaminoglycan chains.