A cold-adapted A/Leningrad/134/47/57 (H2N2) strain, a special donor of attenuation for generation of recombinant vaccine strains safe for children, has been developed and characterized. The genetic markers of this virus (temperature sensitivity of the cold-adapted strain, temperature optimum of the neuraminidase activity) were compared with those of A/Leningrad/134/17/57 (H2N2) strain, a donor of attenuation for a live influenza vaccine for adults. The A/Leningrad/134/47/57 (H2N2) strain was found to have a more marked temperature sensitivity and cold adaptation which correlated with the presence in its genome of multiple temperature-sensitive mutations in genes 1, 2, 5, 7, and 8. When the cold-adapted A/Leningrad/134/47/57 (H2N2) strain was hybridized with epidemic A/Brazil/11/78 (H1N1) and A/Bangkok/1/79 (H3N2) strains, recombinants were isolated which are candidate vaccine strains for a live influenza vaccine for children. The recombinants inherited the antigenic specificity of the current viruses and the temperature specificity and cold-adaptation of the donor of attenuation, A/Leningrad/134/47/57 (H2N2).