The Directigen Flu A assay (Becton Dickinson, Microbiology Systems, Mississauga, Ontario, Canada) is a commercially available immunoassay designed for rapid in vitro recognition of influenza A nucleoprotein. The purpose of this study was to evaluate this assay for detection of influenza virus in nasal secretions of naturally infected horses. The assay was shown to react with representative strains of influenza virus which cause disease in horses and did not react with nasal secretions from uninfected horses kept in isolation. Between 33% and 45% of nasal secretions specimens obtained from clinically diseased horses during influenza epidemics reacted positively in the assay and 95% to 98% of horses not showing signs of disease during influenza epidemics tested negative. In contrast, influenza virus was isolated from only 7% of diseased horses using conventional techniques. Diseased horses which were positive in the Directigen assay had lower pre-exposure influenza antibody concentrations and showed more clinical signs than diseased Directigen-negative horses. This evaluation demonstrates that the Directigen Flu A assay detects influenza virus in nasal secretions of infected horses and is more sensitive than virus isolation.