Paraffin wax embedded formalin-fixed benign breast disease tissue taken from 17 patients (15 with microcystic disease and 2 with fibroadenoma) was studied for the presence of tissue bound prolactin using a rabbit antiserum against human prolactin applied in conjunction with a highly sensitive modified version of the dinitrophenyl (DNP)-hapten sandwich staining (DHSS) procedure. Sections taken from 14 of the 15 cases showing apocrine cystic changes exhibited strong prolactin staining restricted to the cytoplasm of metaplastic apocrine cells lining the cysts. Normal lobules and ducts and blunt duct proliferations were all negative, as were also the two cases of fibroadenoma. In contrast 6 out of 8 cases of breast cancer examined showed heterogenously distributed cytoplasmic staining in the cancer cells. Maximal prolactin staining in the apocrine cells was observed at antiserum dilutions as high as 1:60,000. This compared favourably with a 1:120,000 dilution that gave maximal levels of staining in the prolactotrophs present in serial sections taken from formalin fixed paraffin wax embedded post mortem human anterior pituitaries. In both types of tissues the specific staining was abolished by pre-absorption of the antiserum with human prolactin (10 micrograms ml-1). No staining was observed when the anti-prolactin serum was either omitted or substituted with DNP-labelled normal rabbit serum. Apocrine metaplasia in cystic disease of the breast has recently been found to be associated with an increased breast cancer risk. The strong and selective presence of immunohistochemically demonstrable prolactin in the metaplastic cells may be of significance in view of the hormone's known growth stimulating effect on the breast epithelium.