Glypicans are major cell surface heparan sulfate proteoglycans, the structures of which are characterized by the presence of a cysteine-rich globular domain, a short glycosaminoglycan (GAG) attachment region, and a glycosylphosphatidylinositol membrane anchor. Despite strong evolutionary conservation of the globular domains of glypicans, no function has yet been attributed to them. By using a novel quantitative approach for assessing proteoglycan glycosylation, we show here that removal of the globular domain from rat glypican-1 converts the proteoglycan from one that bears approximately 90% heparan sulfate (HS) to one that bears approximately 90% chondroitin sulfate. Mutational analysis shows that sequences at least 70 amino acids away from the glypican-1 GAG attachment site are required for preferential HS assembly, although more nearby sequences also play a role. The effects of the glypican-1 globular domain on HS assembly could also be demonstrated by fusing this domain to sequences representing the GAG attachment sites of other proteoglycans or, surprisingly, simply by expressing the isolated globular domain in cells and analyzing effects either on an exogenously expressed glypican-1 GAG attachment domain or on endogenous proteoglycans. Quantitative analysis of the effect of the globular domain on GAG addition to proteoglycan core proteins suggested that preferential HS assembly is achieved, at least in part, through the inhibition of chondroitin sulfate assembly. These data identify the glypican-1 globular domain as a structural motif that potently influences GAG class determination and suggest that an important role of glypican globular domains is to ensure a high level of HS substitution of these proteoglycans.