For some time it has been known that influenza viruses contain a number of distinct single-stranded RNA molecules and in the last few years conclusive evidence has been obtained to indicate that both type A and type B viruses contain eight unique polynucleotide chains. This conclusion in the case of type A viruses is based on evidence provided by the results of direct chemical analysis involving RNase T1-oligonucleotide mapping (McGeoch et al. 1976) and nucleotide sequence analyses (J. J. Skehel & A. J. Hay, unpublished data) and by the results of genetic investigations involving analyses of the RNA components of recombinant viruses (Scholtissek et al. 1976; Hay et al. 1977a; Palese, 1977). The results of the former experiments have in addition allowed the identification of other small RNA molecules, detected in some strains of virus as 5′-terminal fragments of one of the largest unique genome segments, and have, therefore, resolved to some extent the ambiguity concerning the number of RNAs per genome.