The spread of West Nile virus (WNV) across the United States into areas with endemic flavivirus activity has complicated serologic surveillance of seasonal virus activity and diagnosis of infected individuals. Here we describe preliminary results from a comparison of serologic assays for flaviviruses: the reference plaque reduction neutralization test (PRNT), enzyme immunoassay (EIA), and a Western blot (WB) in which crude viral lysates were electrophoresed and blotted onto nitrocellulose. Human and chicken sera were tested and compared by each method against WNV and St. Louis encephalitis virus (SLEV). Antibody binding to three viral proteins determined WB interpretation: non-structural protein 1 (NS1), envelope (E), and pre-membrane (prM). WB results for a group of serially collected human plasma samples from WNV seroconverting blood donors were also correlated with transcription mediated amplification (TMA) and polymerase chain reaction (RT-PCR) results. Reactivity with NS1 appeared to be the most useful differentiating marker of WNV and SLEV infection in humans and chickens. Envelope protein was highly cross-reactive and, as indicated by additional results from dengue virus (DENV)-positive human sera, is perhaps useful serologically as a flavivirus group antigen.