From the above data, it is clear that receptor constructs with Q89N substitution have the capacity to increase the binding of some Hi over that of Nm. In previous investigations, we have shown that certain isolates of the two bacteria are capable of competing with the native receptor and that Nm C751 derivatives can displace Ha3 as well as Eagan from the receptor (Virji et al., 2000) . Here we investigated how changes in receptor structure at position 89 may influence this phenomenon. We used receptor dot blot overlay in the presence of varying amounts of the soluble receptor constructs to estimate the ability of C751OpaB and Ha3 to selectively adsorb the receptor. Consistent with the lower affinity of Ha3 compared with C751OpaB/D for CEACAM1 (Virji et al., 2000) , the proportion of the native CEACAM1 that interacted with Ha3 compared with C751OpaB was lower and declined further at limiting concentrations of the receptor (Fig. 6A ). In contrast, Ha3 and C751OpaB had similar affinities for the receptor when the Q89N construct was present in excess (> 0.13 mg ml -1 ; Fig. 6B ). Further, at limiting concentrations of Q89N, the greater affinity of Ha3 was apparent and bound this construct threefold more than the Nm derivative (Fig. 6B ). The data demonstrate the potential influence of structural modulations at the bacterial binding site in changing the colonization profile of the target tissue.