The correlation between the content of individual glycosaminoglycans and the histological patterns are studied on breast tumor tissues. The myxomatous stroma of intracanalicular fibroadenoma contained a large amount of glycosaminoglycans, which were mainly hyaluronic acid. The chondroitin 4- and 6-sulfate level was also high. As the supporting stroma of this tumor became denser and more fibrous, the level of hyaluronic acid content was reduced. In the case of pericanalicular fibroadenoma, glycosaminoglycans were small in amount and the levels of hyaluronic acid and chondroitin sulfate were low, but the ratio of dermatan sulfate content was higher. In the case of gynecomastia, the conent was almost the same as that of pericanalicular fibroadenoma. Scirrhous carcinoma tissues contained a relatively large amount of hyaluronic acid and chondroitin sulfate. No remarkable differences in heparan sulfate content were observed in any one of the breast tumors tested. Dermatan sulfate-chondroitin sulfate copolymers were detected in all the tumors. The presence of dermatan sulfate seemed to have an intimate relation with the fibrogenesis in the interstitial stromal element of the tumor tissues.