Using mutants of fowl plague virus (FRV) which have single temperature-sensitive (ts) mutations in some genes, an analysis was carried out on reisolates from children of 3-6 years, vaccinated with a monovaccine from recombinant strains of influenza type A virus. The recombinants were obtained by crossing of current epidemic strains of subtypes A (HINI) and a (H3N2) with the cold-adapted (XA) ts-donor of attenuation A/Leningrad/134/47/57 (H2N2) from which they, as a rule, inherited 5 ts-mutations in genes 1 (PB2), 2 (PB1), 5 (NP), 7 (M), and 8 (NS). All the reisolates were shown to retain the ts-phenotype. However, in the recombination test some reisolates (most frequently those isolated at late periods of vaccination infection) no ts-mutations could be found in 1-3 genes coding for proteins of the polymerase complex, less frequently for NP and NS proteins but not for M proteins.