We evaluated the antibodies for their ability to block WNV infection in BHK21 cells using a standard plaque-reduction assay 23 . Twelve had strong neutralizing activity that greatly exceeded the potency of immune human γ-globulin, with 50% plaque reduction neutralization titers (PRNT 50 ) below 2 µg, whereas immune human γ-globulin had a PRNT 50 value of 500 µg 5 . The inhibitory activity of two neutralizing antibodies, E16 and E24, was reproduced in J774.2 mouse macrophages and SW13 human adrenal carcinoma cells ( Supplementary Fig. 1 online) and thus was not specific to fibroblasts.