An analysis of ts-mutations in the genomes of native and cold-adapted variants of influenza A/Leningrad/134/57 (H2N2) virus based on the use of fowl plague virus ts mutants was carried out. The recombination test was done by the conventional method in chick embryo fibroblast culture (genes PB2, PB1, PA, NP, NA, M and NS) or cell systems permissive for reproduction of human influenza virus (gene HA). The cold-adapted strain A/Len/17 used for preparation of live influenza vaccine (LIV) for adults was shown to have 4 ts mutations: three in "internal" genes (PB2, NP, and M) and one in gene 4 coding for hemagglutinin (HA). The more attenuated cold-adapted donor A/Len/47 for preparation of similar LIV for children acquired three additional ts mutations: two (PB1 and NS) in "internal" genes and one in gene 6 coding for neuraminidase (NA). The accumulation of ts mutations in the genome of cold-adapter strains was found to be accompanied by a decrease in their pneumotropicity for mice as well as their detectability in different organs of these animals.