A relationship between the kinetics of virus growth in HAE and the level of CPE also suggested that CPE was a consequence of viral replication. This assertion is supported by the fact that trends in viral titers at a given time point are mirrored in AK levels detected 48 hrs later (e.g., compare viral titers at 48 hr pi ( Figure 1A ) to AK measurements taken at 96 hr pi ( Figure 1B) ). Since viral titer and AK levels could be related to the numbers of cells infected and/or the degree of virus replication within individual cells we compared titers of human and avian influenza viruses ( Figure 1A ) to the numbers of cells infected by each virus at the two temperatures over time. Immunodetection of viral antigen in inoculated HAE showed that human and avian influenza virus antigen was not detected 3 hrs pi, indicating that levels of antigen in residual viral inocula were below the limit of antibody detection (data not shown). For A/Victoria/3/75, a few isolated cells were positive for viral antigen by 6 hrs pi at 37uC, but by 24 hrs pi considerable numbers of antigen-positive cells were detected ( Figure 2A ). In agreement with our growth curves in Figure 1A , A/Victoria/3/75 infected slightly fewer cells at 32uC compared to 37uC at 24 hrs pi, but importantly, A/Victoria/3/75 spread efficiently within the epithelium at both temperatures and differences in infection at early time points became less significant over time ( Figure 2A) .