A variety of antigens have been identified on the surface of the malignant cell. However, identical antigens are often found on non-malignant cells of the same or different histological origin, or of a different stage of embryonic development. Many of these tumour-associated antigens appear to be only incidentally expressed on neoplastic cells. Clearly, it would be of great interest to identify cell-surface antigens whose expression is associated specifically with the transformed state and linked directly with the mechanisms responsible for transformation. The detection of activated cellular oncogenes in human and animal cancer cells by the technique of DNA transfection has allowed the isolation of genetic elements which are thought to have a critical role in malignancy. Here, in an effort to identify cell-surface antigens associated with the neoplastic process, we have generated hybridomas which secrete monoclonal antibodies that react specifically with cell-surface determinants found on NIH 3T3 cells transformed by transfection with a group of rat neuroblastoma oncogenes. These antibodies bind to and immunoprecipitate a phosphoprotein of relative molecular mass 185,000 (185 K) from a DNA donor rat neuroblastoma and 13 independent rat neuroblastoma DNA transfectants. There was no antibody reactivity with normal NIH 3T3 cells or with NIH 3T3 cells transformed by various other agents.