Genetic recombination of influenza viruses provides the possibility of immediate reassortment and combination of genes and gene products in a single step. Thus, genetic variants with desirable attributes for vaccine production can be produced by deliberate genetic manipulation of viruses rather than by the empirical” hit or miss” methods of the past. Recombination of a high-yield laboratory strain (A0/PR/8) with a low-yield Hong Kong virus (Aichi strain) produced a high-yield recombinant virus (X-31) of Hong Kong antigenicity suitable for vaccine production. It is proposed that a prefabricated” library” of recombinants might anticipate the mutations which may arise in the future and also that live virus vaccines of greater stability may be produced by recombination of new and old viruses.