Since the demonstration that murine cytotoxic T cells generated during a secondary immune response to influenza A viruses were composed of cross-reactive and subtype-specific populations^1–5, it has become of interest to define the viral components responsible for each pattern of recognition. One approach is by the isolation of individual clones of cytotoxic T cells^6,7 which can then be tested for recognition of natural or laboratory induced virus variants of known genetic composition. We describe here a series of influenza A virus subtype-specific clones raised against the recombinant X31(H3N2)⁸ virus, which contains the haemagglutinin (HA) and neuraminidase (NA) from the 1968 virus, and all its remaining genes from the 1934 virus⁹. We show here that the viral gene responsible for the determinant recognized by these cytotoxic T-cell clones codes for neither the haemagglutinin nor neuraminidase glycoproteins, that it segregates independently of HA and NA during the formation of recombinant viruses, and that it undergoes independent genetic change.