BRCA1 is a tumor suppressor gene which is inactivated by mutation in familial breast and ovarian cancers. Over 300 different disease causing germ-line mutations have been described; 60% are unique to an individual family. This diversity and the large size of the gene lead us to search for a prescreening method for BRCA1 mutations. Since BRCA1 is a nuclear protein in normal cells, but reported by some authors to be cytoplasmic in breast tumor cells of patients with BRCA1 mutation, we evaluated immunohistochemistry as a prescreening technique to identify BRCA1 mutations in patients with familial presentation of breast cancer. Using a monoclonal antibody against the carboxy-terminal region of BRCA1, we performed immunohistochemistry on 18 tumor samples from patients with hereditary breast cancer. Cytoplasmic staining of BRCA1 was observed in 10 cases. Of the 18 tumors, 12 (66%) showed either BRCA mutation or BRCA1 accumulation or both, indicating that BRCA1 function might be lost in breast tumor cells not only through mutation, but also via abnormal cytoplasmic location. The immunohistochemical test used in this study would not be efficient as a pre-screening method of deleterious mutations, but it appeared useful to investigate tumor physiology.