The cytoplasmic site of gene expression and use of virally encoded enzymes is a distinguishing feature of vaccinia virus and other poxvirus vector systems that probably accounts for their consistent ability to express foreign genes derived from a variety of prokaryotic, eukaryotic, and viral sources. This feature, together with their ability to stably integrate and package large amounts of DNA without loss of infetivity, their wide host range, and the development of simple and effective methods for isolating recombinant viruses, account for their diverse use and popularity. During the past year, we have continued to evaluate the highly attenuated MVA strain of vaccinia virus as an expression vector. Because of the inabilty of MVA to complete its replication cycle in human or other mammalian cells, it provides exceptional safety. A recombinant MVA that expresses the influenza virus hemagglutinin and nucleoprotein genes was constructed. Mice immunized with this recombinant virus developed a humoral and cell-mediated immune response and were protected against challenge with influenza virus. The protection was equal to or better than that achieved with a conventional replicating vaccinia virus vector.