The recombination test demonstrated that in the genome of a highly attenuated cold-adapted variant of influenza A/Leningrad/134/47/57 virus five genes coding for nonglycosylated proteins (1, 2, 5, 7, 8) carried ts mutations. Hybridization of this variant with epidemically important strains of influenza H1N1 and H3N2 viruses produced recombinants which inherited hemagglutinin and neuraminidase from the epidemic strains and 5 (H1N1) and 6 (H3N2) genes coding for nonglycosylated proteins from the cold-adapted variant. The genomes of both recombinants were found to contain five genes coding for nonglycosylated proteins which carried ts mutations. High genetic stability of cold-adapted recombinant vaccine strains was established in immunized children.