The immunogenicity of a recombinant virus derived from modified vaccinia virus Ankara (MVA), a host range-restricted, highly attenuated and safety-tested strain, was investigated. Plasmid transfer vectors that provide strong synthetic early/late promoters for the simultaneous expression of two genes as well as a transient or stable selectable marker and flanking sequences for homologous recombination with the MVA genome were constructed. A recombinant MVA containing influenza virus haemagglutinin and nucleoprotein genes was isolated in avian cells and shown to express both proteins efficiently upon infection of human or mouse cells in which abortive replication occurs. Mice, inoculated by various routes with recombinant MVA, produced antibody and cytotoxic T-lymphocyte responses to influenza virus proteins and were protected against a lethal influenza virus challenge as effectively as mice immunized with a recombinant derived from the replication-competent WR strain of vaccinia virus.