In order to establish cell lines which complement the growth of temperature-sensitive (ts) mutants of influenza virus, three RNA polymerase and nucleoprotein (NP) genes each linked to the mouse mammary tumor virus LTR were cloned into the bovine papillomavirus vector DNA. After co-transfection of mouse C127 cells with these recombinant plasmids, a cell line, clone 76, in which the expression of the three polymerase and NP genes could be stimulated by dexamethasone, was established. The clone 76 cells could complement the growth of ts-mutants defective in one of the polymerase subunit genes at the nonpermissive temperature in response to dexamethasone. The results suggest that the simultaneous expression of the three polymerase genes in the same compartment of protein synthesis machinery is required for an efficient complementation of ts-mutant growth.