The authors evaluated the character and frequency of side-reactions after administration of the first dose of the Austrian vaccine against tick-borne encephalitis FSME-IMMUN in 1898 subjects. The mean percentage was 18.9%, in different batches it varied from 4.1 to 24.1%. A statistically higher incidence of reactions was in children under 14 years, as compared with the group above 15 years. While the local reactions were insignificant as regards frequency and severity, the general reactions, i.e. fever, were serious. General reactions were recorded in 13.3% of the vaccinated subjects, in one of the vaccine batches in 19.2%.