A double-blind field trial was performed comparing a subunit influenza vaccine (A/Victoria/75 and B/Hongkong/73) with placebo. A good protection against influenza was induced by the vaccine. On the basis of serological determinations (enzyme immunoassay, EIA) the incidences of influenza A and B infections were reduced within a period from 3 weeks up to 5 months after the vaccination by 88 and 68%, respectively. Three weeks after the vaccination 79% of the vaccines had acquired protective serum antibody levels (greater than or equal to 32 x 10(2) by EIA) against influenza A and 62% against influenza B, while in the control subjects protective antibody levels were measured in frequencies from 4 to 13% in subsequent serum samples. With a few exceptions antibody levels were still present in 5-month samples. Side effects were recorded within the first 3 days following the vaccination. Some minor symptoms like redness and tenderness at the vaccination site and muscle ache were reported more frequently by the vaccines than by the controls, but no more harmful systemic reactions.