Eleven murine monoclonal antibodies (MABs) were produced using human mammary cancer (HMC) cell lines as immunogens: None reacted with normal breast tissue, but each had a distinctive pattern of reactivity with HMC and benign proliferative lesions. Two MABs bound to overlapping epitopes of an M(r) 47,000 cell-surface antigen and were endocytosed: nine bound to secretable intracytoplasmic antigens(s). Four of the MABs immunoprecipitate antigens that upon reduction yielded an M(r) 73,000 moiety. None reacted with components of milk. All antigens were sensitive to trypsin, three to periodate oxidation but none to neuraminidase. One cell-surface-localizing MAB investigated, selectively localized in HMC xenografts.