Differential infectivity between RSV A2, BT2a, BT3a and BT4a. PBECs were infected with RSV A2, BT2a, BT3a and BT4a (MOI of 0.1) during 2 h and washed with PBS. Seventy two hpi, cells were washed with PBS, fixed with 4% paraformaldehyde and stained for RSV F protein (green) and nuclei (blue). Cultures were observed under a confocal microscope. RSV A2 infected considerably more PBECs than the 3 clinical isolates. Original magnification, x10.