By serial subculture of MDCK cells which survived high multiplicity infections with AWBY-140, a weakly cytolytic mutant of influenza virus A/WSN (H1N1), we established a variant cell line (MDCK-L cells) that was uniquely resistant to infection with influenza A and B viruses, yielding 3 to 4 orders lower amount of progeny virus compared with MDCK cells. Competitive polymerase chain reaction revealed that the amount of primary transcript produced in MDCK-L cells infected with 10 PFU/cell of influenza virus A/Aichi was suppressed to 1/100 of that in MDCK cells similarly infected, although the amount of virus adsorbed to MDCK-L cells was 1/4 of MDCK cells. Even when MDCLK-L cells were infected with 40 PFU/cell of Aichi to overcome the lower amount of internalized virus in those cells, the results were the same. The synthesis of v-, c- and mRNAs, as well as proteins of infected A/Aichi was below detectable level in MDCK-L cells, in contrast with MDCK cells, where they were clearly demonstrable by ribonuclease protection assay or polyacrylamide gel electrophoresis.