CD has developed a highly sensitive and specific enzyme immunoassay for the detection of Influenza A nucleoprotein antigen in complex sample matrices derived from both human and veterinary sources. The assay can be completed in less than 1.5 hr. and contains only one wash step. In addition, the test kit incorporates proprietary diluents that are designed to prevent the development of nonspecific signal derived from complex sample matrix effects and/or the nonspecific adsorption of reactive test components which result in improvements in both sensitivity and specificity. The test kit is available in a standard photometric detection format or in a chemiluminescent detection format for enhanced sensitvity. The kit has been tested against a wide variety of influenza A subtypes for sensitivity and potentially interfering viruses and bacteria for specificity.