When Bm-N cells infected with Bm-P12A3C were partial floating, they were detached and collected (about 72 hpi), the cells pellet was freezed and thawed at -70/37uC in PBS for three times and centrifuged at 10,000g for 5 min at 4uC. The supernatant was tested using the sandwich-ELISA method. 96-well plat-bottomed plates (Costar) were coated with the rabbit serum against FMDV overnight at 4uC and blocked with defatted milk powder for 1h. Then the plates were washed five times. FMDV antigen (from the vaccine which had a PD 50 value of 3.6), lysates of Bm-N cells with Bm-P12A3C and BmBacPAK-6 infected were diluted in a twofold series and incubated at 37uC for 1h. Subsequently, the plates were washed thoroughly and guinea pig sera against FMDV was added to each well. The plates were incubated at 37uC for 1 h, and then rabbit anti-guinea pig IgG peroxidase conjugate (Sigma) at 1:10000 dilution was added and reacted at 37uC for 1 h. Substrate (0.05% H 2 O 2 plus orthophenylene diamine) was added, reacted for 15 minutes and stopped by the addition of 1M sulphuric acid. Absorbance was determined at 492 nm.