A monoclonal antibody AF9 (IgG1) was raised against human breast carcinoma by fusing murine myeloma SP2/0 cells with spleen cells from BALB/c mice immunized with cell membrane extracts from primary breast carcinoma. Immunohistochemical staining using the antibody showed reactivity in 19 out of 20 breast carcinoma tissues and in some nonbreast carcinomas such as hepatic, ovarian, lung, colon, and gastric carcinoma tissues. But no staining was observed in either normal tissues or other carcinomas. By immunological treatment, the antigen recognized by AF9 was referred to be glycoprotein with lipid component, and Western blotting revealed that its molecular weight is 51, 56, 67 and 73kd.