Cellular antigens encoded by tumor viruses and some antigens encoded by cellular oncogenes offer advantages as targets for immunotherapy by being inextricably associated with the neoplastic phenotype. For example, monoclonal antibodies (MAb) specific for an antigen encoded by the neu oncogene have a direct inhibitory effect on proliferation of antigen-positive tumor cells. Many of the oncogene-encoded cell surface molecules are growth factor receptors, as are some tumor-associated differentiation antigens (TADAs). Therefore, it is not surprising that their level of cancer specificity is similar. There have been some promising findings from using TADAs as targets for various forms of immunotherapy, and one would expect the results to further improve by targeting to molecules that are more directly involved in cell proliferation and/or in maintaining the malignant state.