Zinc alpha 2 glycoprotein is one of the proteins present in breast cyst fluids, being found at levels 30-50 times its plasma concentration. Using an immunoperoxidase technique the distribution of this glycoprotein has been studied in a range of non-mammary tissues and carcinomas, as well as in normal, benign and malignant breast specimens. The breast cyst fluid protein was detected in all apocrine cells of skin and in the apocrine metaplastic epithelium lining of breast cysts. A progression was apparent from normal to hyperplastic breast in the number of cells reacting, particularly of cystically dilated acini, to a final consistent staining of apocrine-lined cysts. Zinc alpha 2 glycoprotein was demonstrated in 16 of 33 invasive carcinomas, 15 of which were eosinophilic on haematoxylin and eosin staining, and in one of three non-invasive carcinomas. No staining was apparent in other non-mammary tissues and carcinomas apart from weak reactivity of serous cells of the parotid gland. Zinc alpha 2 glycoprotein is, therefore, a reliable immunohistochemical marker of apocrine cell differentiation.