Three intramuscular injections (50 micrograms each) with bacterial plasmid pMV45 carrying nonstructural gene of NS1 protein of tick-borne encephalitis (TBE) virus protected 88% Balb/c mice from lethal challenge with the virus. Antibodies to NS1 nonstructural protein were detected in the sera of vaccinated mice after the challenge. Absence of antibodies to E structural protein indicated absence of manifest infectious process in mice vaccinated with plasmid and challenged with a lethal dose of TBE virus.