The humoral immune response to flaviviruses is mainly directed to the major envelope protein, E, and a glycosylated non-structural protein, NS1. Cell-mediated immune responses, however, appear to be directed mainly against non-structural proteins. Experiments described here show that a defective recombinant adenovirus (Rad51) containing the gene encoding the NS1 protein of tick-borne encephalitis virus can induce a strong protective immune response against several pathogenic tick-borne flaviviruses in an experimental animal model, and can enhance the efficacy of conventional vaccine preparations. A protective immune response against a lethal virus challenge can also be induced by the passive transfer of antibodies, B cells or T cells from animals vaccinated with Rad51. Raised levels of non-neutralizing antibodies and cytokines associated with a T helper cell-type 1 immune response are also observed. These data demonstrate the importance of non-structural viral proteins in the protective immune response against flaviviruses and support the use of non-structural viral proteins as vaccine components.