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 sero-positive 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 acutally 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-specific memory T cells capable of mounting a cytotoxic response when properly challenged in vitro.