The micro-neutralization assay was modified from a previously described procedure [18] . The ascitic fluid was heat inactivated for 30 min at 56uC, and two fold serial dilutions were incubated with 2610 3 tissue culture infective doses 50% (TCID 50 /mL ) of REV-A in a 96-well micro-plate. Four uninfected control wells were included on each plate as control wells. After 2 h incubation at 4uC, 100 mL of CEF cells at 1.5610 5 cells/mL was added to each well. The plates were incubated for 5 days at 37uC and 5% CO 2 . The monolayers were washed with PBS and fixed in icy cold ethanol for 15 minutes. The presence of viral gp90 protein was detected by ELISA with the mAb A9E8. The absorbance was measured at 492 nm with an ELISA microplate reader. The average A492 was determined for quadruplicate wells of virusinfected and uninfected control wells, and a neutralizing endpoint was determined by using a 50% specific signal calculation. The endpoint titer was expressed as the reciprocal of the highest dilution of ascitic fluid with A492 value less than X, where 6= [(average A492 of infected wells) 2 (average A492 of control wells)]/2+ (average A492 of control wells).