Dose–response curves of binding of influenza viruses to plasma membranes from quail, pigs, chickens and ducks. Binding assays were performed using two-fold serial dilution of influenza viruses A/chicken/Hong Kong/G9/97 (H9N2) (A), A/quail/Hong Kong/A28945/88 (H9N2) (B) or A/Panama/2007/99 (H3N2) (C). Data show OD450 values of the mean ± SD of two independent assays run in duplicates: quail trachea (QaTr ♦), quail intestine (QaIn■), pig trachea (PgTr▲), chicken trachea (ChTr×), chicken intestine (ChInҖ) and duck intestine (DkIn●). The results show good correlation values between HA units and OD450 values. A concentration of virus giving 64 HA units was used as the standard concentration for subsequent relative binding assays for these and other viruses, presented in Fig. 5.