Abstract
Under optimal conditions, of high multiplicities of infection and with trypsin included in the medium throughout the incubation period, high yields of infectious influenza A and B viruses (106·5 p.f.u./ml) and of antigenically active haemagglutinin (HA) (1 µg/HA/106 cells) were produced in human diploid MRC-5 cells. Budding virus particles were seen as spherical or short rod-like protrusions on the surface of the infected cells, and also on cell filopodia. Virus-induced cytoplasmic and nuclear inclusions were present in infected cells. This virus-human cell system may be suitable for studies of influenza virus persistence and for production of immunologically active HA antigen.