Primary isolation of type A influenza (H3N2) virus in mammalian Madin Darby canine kidney (MDCK) cells results in a virus with haemagglutinin (HA) identical to that of the virus replicating in the infected individual, whereas similar isolation of virus in the embryonated egg results in the selection of variants with amino acid substitutions in the globular head region of the HA molecule. To determine whether other mammalian and avian host cells routinely used in laboratory isolation of influenza viruses also impose a selective pressure on the replicating virus population, the HA of viruses isolated in several different primary or continuous mammalian cells or avian cells has been characterized. The HAs of H3N2 viruses isolated in monkey kidney LLC-MK2 and primary guinea-pig kidney cell culture were antigenically identical to MDCK cell-grown virus isolated from the same patient. The deduced amino acid sequence over the region of HA1 encoding residues implicated in host cell-mediated sequence variation revealed that the HA sequences of viruses isolated and passaged in these mammalian cell types, and in a human lung continuous cell line (MRC-5), were identical to that of the virus present in the infected individual. In addition, isolation of virus in avian primary chick kidney (CK) cells yielded a predominant virus with HA identical to that of mammalian cellgrown virus and the virus present in the original clinical material. However, passage of CK cell-grown virus in chicken embryos (eggs) resulted in the predominance of viruses with amino acid substitutions in HA, a minority of which resulted in antigenic variation. Since CK cell culture is used in the development of live attenuated influenza vaccines, the sequence identity between CK cell-grown virus and the virus present in the infected individual is reassuring. Nevertheless, subsequent passage of virus strains in eggs, necessary for vaccine production, must be monitored closely.