Effects of baicalein and biochanin on reactive oxygen species (ROS) formation in A549 cells infected with H5N1 strain A/Thailand/1(Kan-1)/04. A549 cells were treated continuously with the investigated flavonoids and/ or N-acetyl-L-cysteine (NAC) starting with a 1 h pre-incubation period prior to infection with H5N1 strain A/Thailand/1(Kan-1)/04 (MOI 0.01). ROS formation was detected at 24 h post infection. A) Chemical structures of biochanin A and baicalein; B) Effects of different baicalein or biochanin A concentrations on H5N1-induced ROS formation. *P < 0.05 relative to virus control; C) Effects of N-acetyl-L-cysteine (NAC) 5 mM on baicalein 40 μM- or biochanin A 40 μM-induced ROS formation in H5N1-infected A549 cells. ‘-’ indicates absence of virus or respective compound, ‘+’ indicates presence of virus or respective compound. *P < 0.05 relative to virus control, #P < 0.05 relative to respective flavonoid alone. Values are presented as mean ± S.D. from three different independent experiments.