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Objective To explore the inhibitory effect of mutant influenza A viruses to the activation of interferon regulatory factor 3 (IRF-3). Methods HEK293 cells were infected with A/FM/1/47,A/HK/1/68, A/HK/1/68-MA20, A/HK/1/68-MA20C and positive control Sendai virus (SV). Whether the slowly moved phosphorylation form Ⅲ and Ⅳ of IRF-3 appeared or not was compared by Western blot in cells infected with these viruses. Wild type of NS1 from A/HK/1/68 and mutant NS1 from A/HK/1/68-MA20 were subcloned into pcDNA3.1-flag respectively. They were transfected in HEK 293 cells respectively. At 16 hours posttransfection, cells were infected with Sendai virus for 8 hours. Whole cell extracts were analyzed by Western blot and then probed with monoclonal flag antibody to check the expression of NS1, or with anti-IRF-3 to observe the inhibitory effects of the wild and mutant NS1 to the activated IRF-3. Luciferase assay was carried out by co-transfection with reporter plasmid, pGL2B with interferon ? promoter, and wild or mutant NS1 cDNA expression plasmid. SV was used to infect these cells after the co-transfection. Results Only less virulent A/HK/1/68-MA20 and positive control SV can activate IRF-3. Activated form Ⅲ and Ⅳ of IRF-3 began to appear 9 hours post infection (h.p.i), and most significant activated IRF-3 appeared 23 and 26 h.p.i. Sequence analysis of NS1 of MA20 revealed that nucleotide position number 94 is mutated from T to C, and amino acid at position number 23 is changed from valine to alanine. Co-transfected with wild type NS1 made form Ⅲ and Ⅳ of IRF-3 almost disappear, but not mutant NS1. In the luciferase functional analysis, wild type NS1 can inhibit the luciferase activity of IFN-? promoter, which was induced by SV, to around 1/10. Again no inhibitory effects was observed of mutant NS1 in the luciferase assay. Conclusion The mechanism that A/HK/1/68-MA20 can activate IRF-3 is that point mutant NS1 abolished the inhibitory function of NS1.