Both a rabbit polyclonal BRCA1 antibody, K-18, and a mouse monoclonal BRCA1 antibody, AP 16, produced nucleolar epithelial cell staining on frozen tissue sections of human infiltrating mammary carcinomas. There was much less BRCA1 antibody staining in normal tissues; however, 2 intraductal tumors and a papilloma, found in proximity to the carcinomas showed considerable nucleolar immunoreactivity. MCF-7 cells fixed in methanol and immunostained with the same two antibodies also revealed nucleolar staining, however, after 4% paraformaldehyde fixation for three minutes, there were many fewer nuclei stained. Antigen retrieval methods on formalin-fixed, paraffin-embedded specimens produced tumor cell cytoplasmic staining with AP 16 and nuclear staining in both tumor and normal epithelial cells with another BRCA1 monoclonal antibody, SG 11.