Breast cancer is the leading cause of cancer deaths among females, and it is estimated that each year, one in ten American women will be newly diagnosed as having the disease. It is therefore not surprising, that a great deal of effort has been made to better understand the biology of breast cancer, and that investigators keep up the search for new tools to better characterize, diagnose and treat these tumours. In this regard, the introduction of the hybridoma technique in 1975 by Kohler and Milstein has lead to an extensive work in the characterization of monoclonal and polyclonal antibodies against breast cancers. A large number of antibodies has been raised to different epitopes present in normal and neoplastic breast tissue; but unfortunately we have yet to find a highly sensitive and specific monoclonal antibody for breast cancer that can successfully be used for scintigraphic detection of nodal metastases and for radioimmunotherapy treatment of this disease. As possible radioimmunodiagnostics, antibodies are known which react with the following antigens: (1) cytoskeletal proteins (2) breast cell products (3) steroid receptors (4) putative tumor-associated antigens (5) oncogene products (6) pregnancy-related products (7) basement membrane antigens (8) degradative enzymes (9) cell receptors for extracellular matrix molecules (10) multidrug resistance gene product (p-glycoprotein) (11) proliferative markers.