The selection of mAb escape mutants was carried out as
reported. X1 virus was incubated
with an excess of mouse ascitic fluid containing mAb at 37 "C for
30 min, and the mixture was inoculated into 10-day-old embryonated
chicken eggs. The presence of mutant virus was confirmed
with hemagglutination assay. Mutant viruses were cloned by
limiting dilution in 10-day-old embryonated chicken eggs.