It has been reported that IBV can infect both ciliated cells and goblet cells in the tracheal epithelium [1, 22] . However, whether basal cells are also susceptible to IBV infection remains obscure. We infected isolated intact tracheas with 50 lL 2575/98 (EID 50 = 10 5 /mL) for 48 h and found that IBV protein expression only colocalized with b-tubulin IV-or mucin 5AC-positive cells (Figs. 4A and 4B) . No viral protein was detected in the K14-positive basal cells of the tracheal epithelium (Figs. 4C and 4D). To further examine whether basal cells are resistant to IBV infection in vitro, 5 · 10 4 ATE cells were also infected with 50 lL of IBV 2575/98 strain (10 5 EID 50 /mL) for 48 h. In vitro results consistently showed that viral proteins of IBV were mainly detected in ciliated cells and goblet cells, but not in basal cells (Figs. 4E-4H). Similar cell tropism results were obtained when IBV 2296/95 or a higher dosage of viral loading was used (data not shown). These in vivo and in vitro experiments clearly delineate the cell tropism of IBV in the avian respiratory tract.