In 1977, a unique event occurred in the epidemiology of influenza when a virus of the influenza A (H1N1) subtype, similar to a virus that had occurred in 1950, reappeared and caused worldwide epidemics but did not replace the prevailing influenza A (H3N2) subtype^1–6. Consequently, the two viruses co-circulated throughout the world^1,7 arid mixed infection of some individuals with both virus strains was detected^8,9, raising the possibility that recombination between the two strains might affect the future epidemiological behaviour of influenza. Serological analysis of virus isolates from influenza outbreaks during the winter of 1978–79, however, failed to detect any antigenic hybrids (H3N1 or H1N2). The investigation described here, was therefore, undertaken to detect recombinants among recent isolates of the H1N1 and H3N2 serotypes, involving genes coding for other than the surface proteins by RNA–RNA hybridisation. We report here the genetic characterisation of recombinants of both antigenic types.