Colon carcinoma cells provide a useful model to study the biochemical processes associated with cell differentiation. Undifferentiated HT29, differentiated HT29MTX(-3) and HT29MTX(-6), and Caco2 human colon carcinoma cells have been used to study the production of proteoglycans and to characterize the glycosaminoglycan structure of the heparan sulphate chains. All the cell lines produce mainly a heparan sulphate proteoglycan that is found partly in the extracellular medium and associated to the cell membrane. The heparan sulphate proteoglycans from the media were purified by ion-exchange chromatography and subjected to structural analysis. The heparan sulphate proteoglycan from differentiated cells is larger and more homogeneous in size than the heparan sulphate proteoglycan from undifferentiated HT29 cells. No differences in protein core structure were observed when cells were labeled with [35S]methionine and the protein cores visualized by gel electrophoresis. Nevertheless, differences in glycosaminoglycan composition were found correlated with the degree of differentiation. The heparan sulphate chains from differentiated HT29MTX(-3) and HT29MTX(-6) cells have a higher sulphation degree than those from undifferentiated HT29 cells. The heparan sulphate from Caco2 cells is the most highly sulphated species. The differences are mainly attributed to O-sulphate groups. The increase in O-sulphation was more pronounced for D-glucosamine 6-O-sulphate than for L-iduronic acid 2-O-sulphate groups.