Inhibitory activity against the huN1 NA of influenza A/New Caledonia/20/99 (H1N1) was detected in 31 of 38 sera tested, and interestingly, inhibitory activity against the avN1 NA proteins of A/Hong Kong/213/03 (H5N1) and A/Vietnam/1203/04 (H5N1) were detected in eight of 38 and nine of 38 individuals, respectively. This evidence concerns the gene XK and influenza.