THERE is remarkably little information about the possible importance of cell-mediated immune responses in the protection of hosts during influenza virus infection. It has recently been found that specific cytotoxic T cells (T_c) can be recovered from the spleens of mice previously inoculated intranasally or intravenously with live virus^1–4 and peak activity occurs about 6 d after virus inoculation. Furthermore, T_c are found in the lungs and the lymph nodes draining the lungs of infected mice in conditions which suggest that these cells might be important in host recovery from this infection⁵. Adoptive transfer of primary or secondary immune spleen cells to mice inoculated intranasally with a lethal dose of A/WSN virus caused a significant reduction of infectious virus levels in the lungs and prevented death⁶. The active cells in the transferred population were T lymphocytes. This report shows that the protective effects conferred by the transferred T cells in these conditions are largely, if not entirely, due to T_c.