X-ray-induced, lymphoblastic, T-cell lymphoma/leukemias from irradiated RF mice were observed to uniformly expressed a 44-kd oncofetal antigen (OFA). The OFA polypeptide was detected by flow cytometry, affinity column SDS-PAGE analysis, and immunoblotting with monoclonal antibody (MAb) 115 prepared against syngeneic mouse fetus. X-ray and ultraviolet (UV) induced murine fibrosarcoma cell lines, used as classic models in radiation biology, were also found to express the OFA, which suggested that the 44-kd OFA was a general transformation marker of tumors. Adult mouse thymocytes and other adult tissues expressed no OFA. The 44-kd polypeptide was located at the surface membrane of the tumors examined. In contrast to other reports, lymphoblastic lymphoma cell lines expressed the OFA as a cross-protective, rather than an individually-specific, tumor-associated transplantation antigen. Pronase treatment removed OFA from the surface of living lymphoma cells, whereas collagenase, neuraminidase, and hyaluronidase did not. The OFA was rapidly reexpressed upon culture of the pronase-treated cells. Taken together, these results suggest that the 44-kd OFA polypeptide described here may provide a useful cell surface marker for future radiation carcinogenesis studies. MAb 115 is a promising reagent for detecting tumor-associated 44-kd OFA, for assessing immunoregulatory perturbations to the OFA caused by radiation damage and for investigating the immunopathology of OFA-associated radiation damage.