The antioxidant N-acetyl-L-cysteine (NAC) had been shown to inhibit replication of seasonal human influenza A viruses. Here, the effects of NAC on virus replication, virus-induced pro-inflammatory responses and virus-induced apoptosis were investigated in H5N1-infected lung epithelial (A549) cells. NAC at concentrations ranging from 5 to 15 mM reduced H5N1-induced cytopathogenic effects (CPEs), virus-induced apoptosis and infectious viral yields 24 h post-infection. NAC also decreased the production of pro-inflammatory molecules (CXCL8, CXCL10, CCL5 and interleukin-6 (IL-6)) in H5N1-infected A549 cells and reduced monocyte migration towards supernatants of H5N1-infected A549 cells. The antiviral and anti-inflammatory mechanisms of NAC included inhibition of activation of oxidant sensitive pathways including transcription factor NF-kappaB and mitogen activated protein kinase p38. Pharmacological inhibitors of NF-kappaB (BAY 11-7085) or p38 (SB203580) exerted similar effects like those determined for NAC in H5N1-infected cells. The combination of BAY 11-7085 and SB203580 resulted in increased inhibitory effects on virus replication and production of pro-inflammatory molecules relative to either single treatment. NAC inhibits H5N1 replication and H5N1-induced production of pro-inflammatory molecules. Therefore, antioxidants like NAC represent a potential additional treatment option that could be considered in the case of an influenza A virus pandemic.