In order to assess the level of protection against a lethal influenza virus infection provided by a primary infection with a virus strain of another subtype, C57BL/6 mice were infected with the sublethal influenza virus X-31 (H3N2) and subsequently challenged with the lethal strain A/PR/8/34 (H1N1). The outcome of the challenge infection was compared with that in mice that did not experience an infection with influenza virus X-31 prior to the challenge infection. The X-31 experienced mice cleared the infection with influenza virus A/PR/8/34 in an accelerated fashion, displayed less clinical signs and a reduction of lesions in the lungs resulting in improved survival rates of these mice compared to the naive mice. The improved outcome of the challenge infection with influenza virus A/PR/8/34 in the X-31 experienced mice correlated with priming for anamnestic virus-specific CD8(+) cytotoxic T lymphocyte (CTL) responses as was demonstrated by the detection of CTL specific for the H-2D(b) restricted NP(366-374) epitope that was shared by the influenza viruses X-31 and A/PR/8/34. Thus previous exposure to influenza A viruses affords partial protection against infection in the absence of virus-neutralizing antibodies specific for the hemagglutinin and the neuraminidase. The implications of these observations are discussed in the light of the current pandemic threat and development of vaccines that aim at the induction of virus-specific CTL.