In man, humoral immunity against influenza viruses (induced by presently available vaccines) is exerted by local and circulating antibodies against two surface antigens of the virions: hemagglutinin (HA) and neuraminidase (NA). Within each major antigenic type of influenza virus (A, B and C) these antibodies are strictly specific of the HA and NA characterizing variable sub-types, which appear in the course of time from an epidemic to the next one. There exists however a cell-mediated facet of immunity: study of the experimental infection of laboratory mice with mouse-adapted viruses has shown that this cellular immunity is type-specific and therefore covers all the sub-types of a given type of virus. This heterologous immunity is primarily mediated by cytotoxic T lymphocytes (CTL) which recognize an internal nucleoprotein (NP) common to all the sub-types. These CTL are able to lyse influenza virus-infected cells and contribute to local production of interferon in the course of infection. Such mechanisms likely play a major role in the natural resistance of the host to this infection. Thanks to the molecular characterization of the NP protein, it is now possible to conceive the design of vaccines endowed with a wider spectrum than those presently used; in association with the latter, it should become possible in the future to stimulate efficiently both facets of anti-influenza immunity.