The present invention is based on the discovery that a high titer reassortant influenza virus is produced in mammalian cell culture by replacing the NS gene of the A/PuertoRico/3/24 master strain with the NS gene of the A/England/1/53 strain. The invention provides influenza viruses and vaccines generated in mammalian cells as well as methods for producing such. The invention further provides an influenza virus master strain and kits for generating reassortant influenza viruses in mammalian cell culture and methods of making and using the master strain.