A live attenuated influenza vaccine candidate was not genetically stable when administered to some children who lacked antibody to surface proteins of the virus. To obtain additional biological information about these revertants, the vaccine strain, the wild-type parental strain, and isolates recovered from inoculated children during a vaccine trial were evaluated in ferret tracheal organ culture for effects on the ciliated epithelium and replication at both permissive and restrictive temperatures. The studies revealed that the vaccine strain destroyed cilia and replicated to high titer at its permissive temperature (33 degrees ) but caused minimal damage and replicated to very low titer at its restrictive temperature (37 degrees C). The wild-type parent destroyed cilia at both 33 and 37 degrees C. Isolates which were no longer temperature sensitive (ts(+)) destroyed cilia at both restrictive and permissive temperatures and grew to high titer. Isolates which retained the ts phenotype behaved as the vaccine strain in this system. The ts(+) virus recovered from volunteers behaved like the wild-type parent, which suggests that these viruses had not merely lost their ts phenotype, but had undergone reversion to wild type. Important information about the genetic stability of temperature-sensitive influenza vaccine strains recovered from volunteers can be obtained by evaluating them in ferret tracheal organ culture.