Examination of Influenza A virus recombinants defined with respect to the derivation of each gene provides a basis for correlating differences in specific genes or combinations of genes with strain related differences in biological activity. Thus far, analyses of such defined recombinants has provided evidence that the unique capacity of WSN virus to produce plaques in bovine kidney (MDBK) cells is a property which segregates with WSN virus neuraminidase. Furthermore, in the same experiments it was shown that WSN virus neuraminidase promotes productive infection in MDBK cells by facilitating cleavage of virus hemagglutinin. In other experiments it was shown that among recombinants of PR8 and HK viruses the property of high yield in embryonated eggs is closely associated with the presence of PR8 derived genes coding for nucleoprotein and M protein. In contrast, comparison of the same recombinants with respect to peak virus titers and lesion production in mice revealed an association of these properties with PR8 virus coded genes for P2, P3, NP, M and NS proteins.