In contrast to A/Victoria/3/75, A/Dk/Eng/62 antigen was detected in only a few cells 24 hrs pi at either temperature. However, it should be noted that antigen-positive cells in en face images are viewed linearly (Figure 2A ) whereas viral titers are shown on a logarithmic scale ( Figure 1A) . Thus, an apparently small difference in titer as is seen at 24 hrs pi between A/Victoria/ 3/75 and A/Dk/Eng/62 at 37uC may correspond to a larger difference in the number of cells positive for viral antigen. While our staining also confirmed previous data that avian influenza viruses infect fewer human airway epithelial cells in comparison to human influenza virus at 37uC (Figure 2A ; [13] ), the limited extent of A/Dk/Eng/62 antigen positive cells at 37uC by 24 hr pi was still unexpected given that titers at this time were slightly greater than those for A/Victoria/3/75 at 32uC. Whether this represents a difference in yield of infectious virus per infected cell between human and avian viruses is presently not clear. Overall, A/Dk/ Eng/62 grew and spread well at 37uC, but was severely restricted for growth at 32uC and antigen positive cells were barely detectable before 48 hr pi for this virus at lower temperature.