A group of 35 adolescents aged 18--22 years were vaccinated intradermally with 0.2 ml of a mixed A (H3N2) + A (Hsw1N1) formolized vaccine and revaccinated with A (Hsw1N1) monovaccine 10 days before an A (H1N1) influenza epidemic. The vaccination had no effect on morbidity or the clinical course of disease. Serology revealed a primary immune response to A (Hsw1N1) and a booster response to A (H3N2). Apparently, prevention of a new influenza subtype by formolized vaccine possessing only the corresponding neuraminidase type is ineffective.