The immunohistochemical reactivities of 69 cases of breast carcinoma were examined on methacarn-fixed, paraffin-embedded sections using eight different monoclonal antibodies which recognize one or a few keratin polypeptides. In the normal breast, the monoclonal antibodies RPN1162, RPN1165 and AE1 stained almost all the luminal cells but not the basal (myoepithelial) cells. The monoclonal antibodies 35BH11, M20, CK5 and CK8.12 stained only a subset of the luminal cells. In contrast, 312C8-1 stained basal cells but not luminal cells. All the tumour specimens reacted with AE1, while over 80% of them also reacted with 35BH11 (57/69), CK5 (57/69) and RPN1165 (55/69); 30% reacted with CK8.12 (21/69) and 16% with RPN1162 (11/69). Basal cell-specific keratin, as defined by 312C8-1, was detected in only 1% of cases (1/69). Monoclonal antibodies to different keratin polypeptides may be of use in the characterization and subdivision of breast cancer.