The bacteriophage T7 RNA polymerase gene was integrated into the fowlpox virus genome under the control of the vaccinia virus early/late promoter, P7.5-The recombinant fowlpox virus, fpEFLT7pol, stably expressed T7 RNA polymerase in avian and mammalian cells, allowing transient expression of transfected genes under the control of the T7 promoter. The recombinant fowlpox virus expressing T7 RNA polymerase offers an alternative to the widely used vaccinia virus vTF7-3, or the recently developed modified vaccinia virus Ankara (MVA) T7 RNA polymerase recombinant, a highly attenuated strain with restricted host-range. Recombinant fowlpox viruses have the advantage that as no infectious virus are produced from mammalian cells they do not have to be used under stringent microbiological safety conditions. Recombinant vaccinia viruses (rVV) have been produced expressing a variety of bacteriophage DNA-dependent RNA polymerases: T7 RNA polymerase (Fuerst et al., 1986), T3 RNA polymerase (Rodriguez et al., 1990) and SP6 RNA polymerase (Usdin et al., 1993). Cells infected with these rVVs can express biologically active products transiently from genes under the control of the appropriate RNA polymerase promoter. The most utilized system is an rVV expressing T7 RNA polymerase, vTF7-3. Translation of the T7 transcripts was inefficient, as only 5-10 % were capped, but inclusion of an internal ribosome entry signal (IRES) sequence, from encephalomyocarditis virus, at the 5' end of the T7-derived RNA transcripts, resulted in mRNAs that were CAP-independent and efficiently translated (Elroy-Stein et al., 1989).In addition to transient expression, a gene under the control of the T7 RNA polymerase promoter can also be integrated into the genome of a second rVV allowing high level expression of the gene product in cells dually infected with vTF7-3 and the second rVV (Fuerst et al., 1987). The vTF7-3 system has been utilized for the expression of RNA per se, e.g. a defective RNA (D-