The strains used in this study have been described previously (Virji et al., 1999; . Nm strain C751 (serogroup A) variants used were C751OpaA, C751OpaB and C751OpaD. The strain MC58 (serogroup B) variant used expressed an Opa previously designated OpaX, which is encoded by the opaB locus. Opa -C751 isolate, which has been shown not to bind to CHO-CC1 (Virji, 1999) and RdCC-, a derivative of THi Rd, known not to bind to CHO-CC1 (M. Virji, unpublished) were used as controls. THi strain Rd is an acapsulate serotype d isolate, Eagan is serotype b isolate and A950002 is a NTHi strain. Hi-aeg strains Ha3, Ha30 and F2087 are all conjunctiva isolates. Nm was grown on brainheart infusion (BHI) agar supplemented with 10% heated horse blood (HBHI). Hi strains were grown on HBHI agar further supplemented with Levinthal base (10 mg ml -1 each of NAD and haemin). All strains were cultured at 37°C in 5% CO2.