Influenza virus polymerase complexes that were expressed in the absence of genomic viral RNA and nucleoprotein were examined for endonuclease activity and transcriptase ability in vitro. Nuclear extracts of cells that express influenza virus polymerase through recombinant vaccinia virus infection did not display specific endonuclease activity in vitro. This polymerase presumably represents an early form of enzyme present in infected cells prior to ribonucleoprotein assembly. Upon addition of a virus-like model RNA template, containing the partially complementary sequence found at the ends of viral RNA, endonuclease activity is stimulated in a concentration-dependent and sequence-specific manner. Once stimulated, the polymerase is able to elongate from the added viral template. Thus, addition of viral template is required for polymerase activity, while the presence of nucleoprotein is not required for limited transcription. Also, full activation of this recombinant viral polymerase is dependent on the presence of both the 3' and 5' ends of the viral genome, as model RNA containing either end alone could not effectively trigger the endonuclease.