The present work reports the establishment of an antigen-specific cytotoxic T cell lymphoma line after immortalization with a murine leukemia virus. Lymph node cells from mice bearing a transplanted syngeneic MCA sarcoma were infected in vitro with radiation leukemia virus and injected intrathymically into cogeneic recipient mice. Some lymphomas of donor origin were established as permanent continuous cell lines in vitro. One of them, NS8, expressed Thy-1.2, Lyt-1, Lyt-2 and peanut agglutinin surface markers. These cells were cytotoxic in vitro for the tumor cell line corresponding to the immunizing MCA sarcoma. No significant cytolytic activities against other syngeneic or allogeneic sarcomas or lymphomas, nor against the mastocytoma P815 were observed. After several months of in vitro propagation, the specificity of the cytotoxic activity had degraded and the level of Lyt-2 or peanut agglutinin receptors dropped. These characteristics were restored with a single in vivo passage. Thus, murine leukemia virus can be used to immortalize antigen-specific cytotoxic T cells.