Mouse monoclonal antibodies (Mabs) against the human breast cancer cell line MCF-7 were obtained by fusing spleen cells from immunized mice with SP2/0 myeloma cells. The Mabs obtained show a high degree of specificity for epithelial cells. They react in a heterogeneous way with neoplastic and non-neoplastic tissues. Mabs 5D10, 2B4, and 3B7 recognize the same antigen (MW 80,000-90,000) in fresh tissue and in paraffin embedded sections. Mab 11F9 recognizes another antigen which can only be detected in unfixed tissue and not in paraffin sections. A preliminary study suggests a glycolipid nature of all recognized antigens. At least one of the monoclonal antibodies (5D10) developed can be used in an in vitro and in vivo model for the study of the invasiveness of MCF-7 cells.