West Nile virus (WNV) causes severe neurological disease and fatalities in both human and animal hosts. There is currently no effective vaccine or antiviral drug to protect against WNV infection. West Nile virus NS3 protease, essential for the proteolytic processing of the viral polyprotein precursor, is a promising target for the development of anti-WNV drugs. The SensoLyte® 570 West Nile Virus Protease Assay Kit provides a convenient homogeneous assay for high throughput screening of WNV protease inhibitors and for continuous assay of NS3 protease activity using QXL™570/5-TAMRA FRET substrate. Upon cleavage into two separate fragments by the WNV NS3 protease, the fluorescence of 5-TAMRA can be monitored at excitation/emission = 540 nm/575 nm.