Effects of baicalein and biochanin A on reactive oxygen species (ROS) formation and H5N1 replication in primary human monocyte-derived macrophages in combination with N-acetyl-L-cysteine (NAC). Macrophages were infected with H5N1 strain A/Thailand/1(Kan-1)/04 (MOI 1). Drugs were continuously present starting with a 1 h pre-incubation period. Virus titres were determined 48 h post infection. A) Effects of flavonoids 40 μM and/or NAC 5 mM on H5N1-induced ROS formation, ‘-’ indicates absence of virus or respective compound, ‘+’ indicates presence of virus or respective compound. *P < 0.05 relative to flavonoids alone; B) Effects of baicalein 40 μM or biochanin A 40 μM in the presence or absence of NAC 5 mM on H5N1 titres. ‘-’ indicates absence of virus or respective compound, ‘+’ indicates presence of virus or respective compound. Values are presented as mean ± S.D. from three different independent experiments.