The immunohistochemical reactivity on frozen sections of diverse benign and malignant epithelial proliferations of human breast tissue from 156 patients was examined using antibodies to different cytokeratins. Antibodies recognizing cytokeratins 18 and 19 reacted with luminal epithelial cells but not with myoepithelial cells of normal mammary gland, cystic disease, adenosis, papilloma, and fibroadenoma or with a subpopulation of proliferating cells in sclerosing adenosis and epitheliosis. These antibodies reacted with the tumor cells of all in situ and invasive carcinomas. KA1 antibody, which by one- and two-dimensional gel electrophoresis and immunoblotting was shown to bind preferentially to cytokeratin 14 in a complex with cytokeratin 5, reacted with the nonproliferating myoepithelium of normal gland, cystic disease, adenosis, papilloma, fibroadenoma, and in situ carcinoma; it also reacted with a subpopulation of proliferating cells in sclerosing adenosis and epitheliosis (papillomatosis) but was negative with the tumor cells of all preinvasive and most invasive carcinomas. In adenotic and epitheliotic proliferations, groups of cells were identified that reacted strongly with KA1 antibody in addition to antibodies to cytokeratins 18 and 19. The data are discussed with respect to epithelial cell heterogeneity in the breast. We show that by using such antibodies, benign epithelial proliferations are clearly distinguished from carcinomas.