Antibody-based therapies have better specificity and thus improved efficacy over standard chemotherapy regimens, which result in extended survival and improved quality of life for cancer patients. Because antibodies are viewed as too large to access intracellular locations, antibody therapy has traditionally targeted extracellular or secreted proteins expressed by cancer cells. However, many oncogenic proteins are found within the cell (such as intracellular phosphatases/kinases and transcription factors) and have therefore not been pursued for antibody therapies. Here, we explored the possibility of antibody therapy or vaccination against intracellular proteins. As proofs of concept, we selected three representative intracellular proteins as immunogens for tumor vaccine studies: PRL-3 (phosphatase of regenerating liver 3), a cancer-associated phosphatase; EGFP (enhanced green fluorescent protein), a general reporter; and mT (polyomavirus middle T), the polyomavirus middle T oncoprotein. A variety of tumors that expressed these intracellular proteins were clearly inhibited by their respective exogenous antibodies or by antigen-induced host antibodies (vaccination). These anticancer activities were reproducibly observed in hundreds of C57BL/6 tumor-bearing mice and MMTV-PymT transgenic breast tumor mice. Our in vivo data suggest that immunotherapies can target not only extracellular but also intracellular oncoproteins.