Experiments have been conducted to determine the role played by immune T cells in the regression of EB‐virus‐induced transformation which is exclusively seen in leukocyte cultures from seropositive donors. Kinetic studies suggest that, In virus‐infected cultures from such donors, a population of T cells proliferates within the first 2 weeks apparently in response to the appearance of virus‐infected B cells. This proliferation continues to some extent during the period of regression. Nonspecific induction of T‐cell proliferation by PHA did not induce regression in virus‐infected cultures from seronegative donors and actually prevented the regression in seropositive donor cultures. T cells harvested from seropositive donor cultures 11–14 days post infection were generally much more inhibitory to the growth of the autologous EB‐virus‐transformed cell line than were T cells either freshly prepared from whole blood or harvested from corresponding uninfected cultures; this inhibitory activity was either absent or much diminished when assayed against allogeneic target cell lines. The results suggest that virus‐infected individuals possess a pool of virus‐specific memory T cells capable of mounting a cytotoxic response when properly challenged in vitro.