The NA inhibition test using large molecular fetuin substrate is believed to be the most specific method to detect antibodies against influenza virus neuraminidase. To avoid cross-reacting with hemagglutination-inhibition antibodies we have prepared several reassortant A/H7N1 viruses containing the NA of A/California/07/09(H1N1)pdm, А/New Caledonia/20/1999(H1N1) or A/Vietnam/1203/04(H5N1), and the hemagglutinin derived from A/equine/Prague/1/56(H7N7) influenza virus. Adjustment and validation of the NI test with the reassortant viruses included the determination of the viruses working concentration and selection of optimal conditions for the enzymatic reaction.