The present invention relates to novel MDCK cells which can be to grow viruses, e.g., influenza viruses, in cell culture to higher titer than previously possible. The MDCK cells can be adapted to serum-free culture medium. The present invention further relates to cell culture compositions comprising the MDCK cells and cultivation methods for growing the MDCK cells. The present invention further relates to methods for producing influenza viruses in cell culture using the MDCK cells of the invention.