Heparan sulfate at cell surfaces and in the extracellular matrix regulates cell proliferation and adhesion by binding to growth factors and matrix proteins via structurally specific oligosaccharide domains. We have used the hormonally regulated mouse mammary carcinoma cell line S115 as a model to elucidate the effect of malignant transformation on the structure of heparan sulfate. When cultured in the presence of testosterone, S115 cells form tumor cell colonies in soft agar and exhibit fibroblast-like morphology; withdrawal of testosterone results in a loss of the tumorigenic capacity and a switch towards epithelial morphology. Metabolically 35SO4-labeled heparan sulfate was isolated from testosterone-treated and non-treated S115 cells and subjected to structural analysis. We found that the testosterone-dependent malignant transformation was associated with reduced sulfation of heparan sulfate due to a approximately 40% decrease in the amount of GlcN6S units. By contrast, no significant differences were observed in the amounts of 2-O-sulfate or N-sulfate groups. The reduced 6-O-sulfation of GlcN units in heparan sulfate from transformed S115 cells led to a marked decrease in the amount of trisulfated IdoA2S-GlcNS6S units (IdoA, L-iduronic acid), implicated in many heparan sulfate-protein interactions.