A simple method of assaying anti-influenza neuraminidase antibodies in human sera was described. Suitable antigenic hybrid viruses were adsorbed to sheep erythrocytes, which were then incorporated into agarose gels. When sera were introduced into wells cut in the gels, zones of hemolysis were observed in the neighborhood of those containing neuraminidase antibodies. There was a direct relationship between zone size and antibody titer. No purification of adsorbed viruses was necessary. The test was rapid, required very simple reagents, gave results that agreed well with those given by conventional techniques, and appeared to be the most sensitive of four methods evaluated. Studies of cross-reactions by hyperimmune sera against homologous and heterologous neuraminidases and of absorption of neuraminidase antibodies from human sera indicated a high degree of specificity. The technique seems to be suitable for large-scale epidemiological investigations.