Ferret tracheal organ cultures prepared from animals previously infected intranasally with influenza A virus required approximately 130 times more homologous virus (A/PR/8/34(HON1) or A/Port Chalmers/1/73 (H3N2)) to become infected in vitro than similar cultures from normal ferrets. Also, these cultures from convalescent ferrets required 9 times more heterologous virus (A/PR/8/34(HON1) or Sendai) to become infected in vitro than similar cultures from normal animals challenged in vitro with the hererologous virus. We conclude that these tracheal rings are specifically immune. Once tracheal rings are infected, they continue to shed viruses for at least 60 days, the longest period any cultures were kept. Virus sheeding in the intact ferret lasts normally 5-7 days. Thus recovery in the intact ferret seems to be dependent upon factors which are not present, or at least not functional, in the tracheal explant. This is consistent with the hypothesis that recovery is dependent upon systemic rather than local phenomena. Bladder tissue from normal and previously infected ferrets was also cultured and challenged with homologous and heterologous virus. The bladder from previously infected ferrets exhibited specific immunity, although the immunity was more varible.