We have previously demonstrated in several species that sarcomas, lymphomas, and carcinomas express a common Ag that cross-reacts with midgestation fetal cells. We also produced a mAb to that protein and characterized it as a 44-kDa glycoprotein. The cross-reactive immunity induced by immunization with tumor or fetal cells expressing the oncofetal Ag (OFA) can be adoptively transferred with cell populations containing T lymphocytes. The experiments discussed within this paper describe the establishment and characterization of two types of T lymphocytes induced by immunization with syngeneic tumor cells in two mouse strains. We find that five of the eight cloned T cells derived from spleens of BALB/c mice that had been immunized with MCA1315 fibrosarcoma cells are specific for an Ag shared by MCA1315 and MCA1321 cells. The other three clones are specific for an Ag present on MCA1315 but not on MCA1321. Also, none of the clones were reactive with the BALB/c plasmacytoma MOPC-315, which does not express OFA. We also find that 75% of the RFM T cell clones from spleens of RFM mice immune to the RFM thymoma 5T show a 5T-specific proliferative response. One of the four clones, however, responds to both 4T and 5T RFM thymoma cells. The BALB/c and RFM cross-reactive clones specifically respond to purified 44-kDa OFA derived from MCA1315 fibrosarcoma cells in the presence of syngeneic irradiated spleen cells and IL-2. All of the clones from both strains of mice, be they tumor-specific transplantation Ag specific or OFA specific, are CD4+, CD3+, alpha beta TCR+ T cells that secrete IFN-gamma on Ag stimulation.