Gross cystic disease of the breast is a benign pre menopausal disorder in which cysts are a predominant pathological lesion. These cysts appear to be formed from excessive apocrine cystic secretions. This fluid is composed of several glycoproteins including a unique 15 kD monomer protein, GCDFP15. Cytosolic analysis of normal tissue specimens from all major organs has demonstrated GCDFP15 in apocrine epithelia, lacrimal, ceruminous and Moll=s glands and in numerous serous cells of the submandibular, tracheal, bronchial, sublingual and minor salivary glands. Cytosols from breast carcinoma specimens also contain GCDFP15 in a wide range of concentrations. The concentration is highest in more differentiated carcinomas. Immunohistochemical staining of normal breast tissue for GCDFP15 shows only a few positive individual epithelial cells within lobules and small ducts. Positive staining has also been observed in fibroadenomas with areas of apocrine metaplasia. GCDFP15 and prostate specific antigen are co-expressed in androgen receptor-positive breast tumours. This antibody may be of use in the identification of breast carcinoma, salivary duct carcinoma and apocrine epithelia.