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Sera from 341 individuals living in the distribution area of the tick Ixodes recinus were tested for tick-borne encephalitis (TBE) antibodies by HAI and gel diffusion. Kaolin treatment was unreliable for the removal of non-specific HAI inhibitors. Seven sera positive after this treatment were shown to be negative after acetone extraction/flotation centrifugation. The antibody prevalence rate was 19.6%. Seventy-one % of the sera had titres greater than or equal to 40. The prevalence rate decreased with age. Some sera with low HAI titres could be confirmed by a sensitive Ouchterlony technique, while some with high titres could not, even after ten-fold concentration. Clinical information obtained retrospectively regarding patients with high antibody titres revealed some cases consistent with a TBE virus infection. Antibody prevalence rates indicate that TBE virus is more active than Uukuniemi and Kemorovo group virus in tick-infested areas. Mixed foci of these viruses have been indicated by serological findings and virus isolations.