The stability of cold adaptation, temperature-sensitivity, and marker mutations that are typical of attenuated influenza A and B viruses - master donor strains and their based reassortant vaccine strains was studied. After 5 sequential passages in chick embryos (CE) at resolving temperatures of 32 and 37°C, the master donor strains and vaccine viruses retained their adaptability and temperature sensitive phenotype. Passage at the temperatures maximally permissible for viral reproduction (39 and 38°C for influenza A and B viruses, respectively, aborted infection just during the second passage. After a series of passages at all study temperatures), there was neither loss or nor substitution of the marker mutations typical of the cold-adapted and temperature-sensitive phenotype of attenuated viruses. The study supports the high genetic stability of attenuated cold-adapted influenza A and B viruses during CE passage not only at the optimum, but also at elevated incubation temperatures.