A fetuin-latex agglutination test (FLAT) inhibition procedure was developed for measuring anti-neuraminidase antibody titers in sera. The use of recombinant strains with equine haemagglutinin (HA) and the pertinent N1 or N2 neuraminidases ensured that the presence of anti-HA antibodies would not interfere in the test. The titres obtained were compared with those measured by the inhibition of enzyme activity (EAI). It was found that FLAT inhibition is as specific as EAI in assessing anti-neuraminidase titres in hyperimmune rabbit sera. There was no appreciable inhibition of N1 strains by N2 antibodies and vice versa. As regards sensitivity, FLAT inhibition is five- to 20-fold less sensitive than EAI. With human sera from vaccinated subjects, EAI is more efficient than FLAT inhibition in measuring increases in antibodies to N2 strains whereas for N1 strains, FLAT inhibition is more efficient. With convalescent sera, the number of seroconversions detected by both techniques is essentially similar. The possibility that the antibodies which inhibit FLAT have a more restricted specificity than those which inhibit EA is discussed.