Sera collected from Australian pelagic birds specifically inhibited the neuraminidase of the Asian/57 (H2N2) strain of human influenzavirus. Neuraminidase inhibition titres of some sera were high and the avidity of the inhibitor was comparable to that of specific antibody. The neuraminidase of A/Hong Kong/1/68 (H3N2), which has undergone considerable antigenic drift in man since 1957, was inhibited to a lesser extent by the bird sera, while the neuraminidases of the A/BEL/42 (H0N1) and A/FM/1/47 (H1N1) strains were not inhibited at all. The inhibitor could be removed from the sera by adsorption with A/57 virus particles, but not by particles of A/BEL or A/FM1 viruses. These results suggested that the inhibitor in the bird sera was specific antibody. The antibodies to A/57 neuraminidase were found in sera from wedge-tailed shearwaters (Puffinus pacificus) and noddy terns (Anous minutus) nesting on islands off the north-east coast of Australia. They were not found in sera from bridled terns (Sterna anaetheta) or brown gannets (Sula leucogaster) nesting on the same islands. Antibodies to A/57 neuraminidase were also detected in sera from short-tailed shearwaters (Puffinus tenuirostris), which migrate around the Pacific Ocean, suggesting that these birds may be responsible for bringing avian influenzaviruses from areas in the Northern Hemisphere into Australian coastal waters.