Blood was obtained from the retro-orbital plexus with a heparinized capillary tube, collected in an Eppendorf tube, and centrifuged, and serum was stored at −20 o C. RSV G protein-specific antibody titers in immunized mice were measured by a direct enzyme-linked immunosorbent assay (ELISA). Briefly, 96-well plates were coated overnight with 100μl/well of 0.5μg/ml of purified G protein fragment diluted in PBS and then blocked with PBS containing 1% skim milk and 0.05% Tween-20 for 2 h. Sera were then added in serial dilutions and incubated for 2 h. The plates were washed five times with PBS containing 0.05% Tween 20 and incubated for 30 min with various dilutions of horseradish peroxidase-conjugated affinity-purified rabbit anti-mouse total IgG, secondary antibody (Zymed Laboratories, San Francisco, CA). The plates were washed five times and developed with 3,3',5,5'-tetramethylbenzidine, and the reaction was stopped with 1 M H 3 PO 4 and analyzed at 450 nm with a Thermo ELISA plate reader. The wells receiving no serum were used to calculate cut-off values.