Volunteers were immunized once, twice or three times in 1990-1992 with commercial trivalent inactivated influenza vaccine and monitored for haemagglutination inhibition (HI) antibodies. The antibodies that already existed when the subjects entered the study derived from natural infections. Immunizations in the second and third years were necessary to maintain the geometric mean titres of antibody and protection rates at the level recorded after the first vaccination. Negative correlations between prevaccination antibody titres and mean fold antibody increases were noted in most instances analysed. Moreover, at each individual prevaccination titre level the mean fold antibody increases and even postvaccination mean titres were higher after the first than after subsequent vaccinations, suggesting that the HI antibody responses might be affected by vaccine-induced pre-existing antibody more than by the same titres of antibody derived from natural infections. This was most obvious for antibody to the H1N1 subtype virus, A/Finland/164/91. In immunization with B/Yamagata/16/88, anamnestic response of antibody to B/Finland/150/90, which belongs to the antigenically distinct lineage of B/Victoria/2/87-like viruses, was more frequent in the first than in subsequent years. This is in contrast to homologous antibodies, which increased significantly after the second vaccination.