Using isolated nuclei prepared from influenza virus-infected HeLa cells, factors affecting the synthesis of two species of positive-sense RNA transcripts, i.e., mRNA and cRNA (complementary RNA to vRNA) were analyzed. In the presence of low concentrations of salt, both mRNA and cRNA were synthesized, whereas in the presence of high concentrations of salt, mRNA was synthesized predominantly. Salt-extracts of nuclei (NE) mainly produced cRNA while mRNA was a major product synthesized by salt-treated nuclei (delta N). In the presence of high concentrations of salt, the NE produced mRNA instead of cRNA. After centrifugation of the NE, the precipitates (NEP) predominantly produced mRNA while the supernatant (NES) alone exhibited a low level of cRNA synthesis activity. With the addition of the NES fraction, mRNA synthesis by the NEP was switched to cRNA synthesis. Glycerol gradient centrifugation of the NES fraction in the presence of high salt yielded vRNA-RNA polymerase complexes that catalyzed mRNA synthesis. These observations indicate that a regulatory factor(s) that can be dissociated from vRNA-RNA polymerase complexes upon exposure to high ionic strength is involved in the switch from mRNA to cRNA synthesis. This activity was not detected in nuclear extracts prepared from uninfected cells, suggesting that such a factor(s) is either encoded by the virus genome or induced by virus infection.