The frequency and intensity of undesirable side effects after protective vaccination against influenza with a commercially available vaccine were studied. 82 gainfully employed persons of both sexes were questioned in respect of local reactions such as weal and flare reactions, itching, sensation of heat, tenderness on pressure, and impeded movement, as well as systemic reactions e.g. elevated temperature or fever, profound sweating, headache, malaise, and insomnia. In 61 of 82 questioned persons local reactions occurred within 24 hours after vaccination, mainly weal and flare reactions and tenderness on pressure. 10 persons had systemic reactions, chiefly headache and fever, 39 of the questioned persons who reported on undesirable effects considered these impairments to be slight, whereas 28 of the group did not feel that the impairments disturbed or upset them in any way.