Abstract: It is known that the human influenza virus strain A/Puerto Rico/8/34 grows particularly well in eggs and that reassorted viruses having it as a parent may also grow well in eggs. It has now been found that certain reassortants of A/PR/8/34 and equine influenza viruses, namely those which comprise the RNA7 segment from A/PR/8/34, will grow in cell culture, even though the parent equine influenza vius will not.Thus the specification describes and claims: reassorted viruses comprising genes for surface antigens of equine influenza viruses and the RNA7 segment from A/PR/8/34; methods of obtaining such viruses by reassortment; methods of propagating such reassorted viruses in cell culture, especially Vero cells; vaccines against equine influenza comprising such reassorted viruses; and methods of vaccinating equines against influenza.