We have previously documented the development and subsequent disappearance of cytolytic activity mediated by lymphocytes from lymph nodes draining Moloney sarcomas destined either to regress or grow progressively. We now report that these tumour-draining lymphnode cells (LNC) that were no longer cytotoxic, spontaneously regenerated peak levels of killing after culture in vitro for 4 days in the absence of exogenous tumour antigen. Cytolytic activity, which was antigenically specific, was mediated by T lymphocytes. Resurgence of cytolytic activity in vitro was accompanied by proliferative changes (DNA synthesis, blast transformation, cell division) which peaked on the 3rd day of culture. Although normal, nonimmune LNC underwent quantitatively similar proliferative changes in culture, the killing that developed was weak and antigenically nonspecific. Transfer of cultured, tumour-draining LNC to immunologically compromised, syngeneic mice conferred complete protection from Moloney sarcoma progression. Adoptive transfer could be delayed for 6 days after tumour induction without loss of protection. These results suggest that there exists in Moloney sarcoma-bearing mice a mechanism that limits the differentiation of pre-killer cells into cytolytically active T lymphocytes, and that such inhibition is eliminated when LNC are explanted into culture. © 1978, The British Empire Cancer Campaign for Research. All rights reserved.