The involvement of inoculated virus antigens in the induction of target susceptibility to cytotoxic T lymphocyte (CTL)-mediated lysis was investigated using heat-inactivated influenza virus, PR8 strain, and various inhibitors in comparison to the cases for live or ultraviolet (u.v.)-irradiated influenza and Sendai viruses. Induction of target susceptibility with heated PR8 was not inhibited by cycloheximide and actinomycin D as in the case of u.v.-irradiated Sendai virus, whereas live virus and u.v.-irradiated PR8 were inhibited under conditions which suppress protein synthesis. Induction of target susceptibility with the live and inactivated PR8 tested was suppressed in the presence of chloroquine, contrary to the case of Sendai virus, and was dependent on the cleavage type of influenza virus haemagglutinin. These findings suggest that the viral target antigens recognized by CTL in heated PR8-coated targets came from inoculated virus proteins, whereas those in PR8-infected or u.v.-irradiated PR8-coated targets involved newly synthesized viral proteins. The former viral target antigens seem to be transferred or processed from the endosome, depending on low pH fusion in the endosomes into which they were engulfed. In this point, the induction of viral target antigens with heated PR8 was different from that induced by u.v.-inactivated Sendai virus. Targets made with heated PR8 were recognized by cross-reactive CTL over the HA subtype.