36 young adult volunteers were given either virus, or inactivated surface-antigen influenza vaccine, containing influenza A (H1N1) antigen. Their peripheral-blood lymphocytes were analysed to determine whether cytotoxic T lymphocytes (CTL) were induced by these vaccines. All three vaccines induced HLA-restricted T-lymphocyte responses specific for influenza A virus. This CTL response was found in 28 of 30 volunteers who showed antibody responses and in 3 of 6 who did not develop antibodies. Virus infected target cells showing certain HLA antigens (A1, A9, and B8) were less susceptible to lysis by HLA-matched cytotoxic T effector cells than target cells which matched effectors at other HLA loci.