Replication of influenza A and B viruses in human diploid cells.

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 (10(6 . 5) p.f.u./ml) and of antigenically active haemagglutinin (HA)(1 microgram/HA/10(6) 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.