Abstract: Five isolates of newcastle disease virus prevailed in 2001-2003 in China were collected. These isolates were purified by CEF plaque assay and replicated in SPF embryos. The fusion protein （F） gene and hemagglutinin-neuraminidase （HN） genes of these viruses were cloned and sequenced. La Sota vaccines with type ⅡF gene was used to immune SPF group in the isolator. The NDV antibody titers were detected every seven days and chickens including SPF control group were challenged by living isolate virus and standard F48E9 strain after two weeks. It is important to autopsy the sickness and dead chickens and ensure the vaccines in order to analyze the immune failure in farm process.

Key words: newcastle disease, vaccine, protection, F gene, HN gene