Human peripheral blood lymphocytes have been sensitized in vitro to influenza virus antigen. After an induction period of 4–14 days, cytotoxic cells which lyse autologous influenza virus‐infected lymphoid cells could be demonstrated. The cytotoxic cell is a T lymphocyte which shows specificity for sensitizing influenza virus type A or B. It cannot distinguish between major subtypes of influenza A virus. The use of virus‐infected normal lymphoid cells as target cells overcame the difficulties of nonspecific killing encountered with some transformed cells.