BRCA1 is a familial breast and ovarian cancer susceptibility gene and encodes proteins that function as tumor suppressors in human breast cancer cells. To elucidate the biological function of BRCA1, knowledge of cellular localization is needed. This can be achieved by using specific antibodies, so in a first step, we characterized by Western blot analysis the rabbit polyclonal antibodies (K-18) and (D-20) raised against the amino-terminus of human BRCA1 protein, and the polyclonal antibodies (C-20) and (I-20) raised against the carboxy terminus of human BRCA1 protein. The 220-kDa band corresponding to BRCA1 protein was recognized by the four tested antibodies in two mammary carcinoma cell lines (HBL100 and MCF7).