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 degrees 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 degrees 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.