Methods for determining the prognosis of human carcinomas utilizing a marker specific for malignant cells of aggressive cancers. Cell lines have been produced that secrete monoclonal antibodies useful in detecting such markers, including a 51,000-dalton keratin protein, specific for myoepithelial cells, e.g., in tissue culture. Pharmaceutical compositions containing these antibodies, which can be in combination with cytotoxic agents and the use of such compositions in the management of carcinomas are included.Prior to filing of this patent application, the continuous transformed cell lines described herein was deposited in the American Type Culture Collection and designated Accession No. HB9288.