Acid and neutral mucopolysaccharides are present in luminal secretions of benign human breast lesions including fibroadenoma, fibrocystic mastopathy, and sclerosing adenosis. Such material bas been demonstrated also in the intraepithelial, extracellular, or luminal secretion of infiltrating duct carcinomas. In a chronological series of infiltrating carcinomas examined for mucin, 95 percent revealed such secretion in the primary lesion. The available metastases contained similar secretion. The histochemical properties of the acid mucopolysaccharides in these lesions and in colloid carcinomas are those of enzymatically digestible sialomucin.