Genetic recombination between different strains of influenza A virus takes place readily under laboratory conditions (reviewed by Kilbourne, 1963) and this fact has recently found practical application in techniques for ‘tailor making’ antigenic ‘hybrid’ strains of influenza, the antigenic composition of which is pre-determined by the appropriate choice of parent strains (Tumova & Pereira, 1965; Kilbourne et al. 1967; Webster, 1970; McCahon & Schild, 1971). Recombination has also been suggested as a method for the production of strains with high growth capacity in the embryonated egg for use in the preparation of inactivated influenza vaccines (Kilbourne & Murphy, 1960) and recently Kilbourne (1969) has reported the isolation of a recombinant virus (X-31) using A0/pr8/34 virus and A2/hong kong/68 as parent viruses. X-31 was considered as appropriate for vaccine production since it exhibited the high yielding growth characteristics of A0/pr8 but was antigenically identical to the current epidemic virus A2/hong kong/68.