In this study, determination of the antigenic peptides recognized by current aromatase antibodies through epitope mapping, and taking into consideration of the interference with aromatase immunohistochemical staining by reductase, we demonstrated that MCA 677 is a suitable antibody for an assessment of intratumoral aromatase activity in breast cancer patients for use in clinical management decisions. The gene discussed is CYP19A1; the disease is breast carcinoma.