The N-domain specific monoclonal antibody (mAb) YTH71.3 has been shown to require Gln-89 and Ile-91 for receptor recognition and substitution of these amino acids had no effect on the binding of the polyclonal anti-CEACAM antibody A0115 (Virji et al., 1999) . The novel receptor constructs produced in the current studies were first examined for their ability to bind to YTH71.3, A0115 as well as Kat4c. The latter mAb recognizes the A and B domains of the receptor (Jones et al., 1995) . AO115 and Kat4c bound to various receptor constructs and to the native NA1B-Fc molecule with equal efficiency. For YTH71.3, Leu but not Thr at position 91 supported binding to the same extent as Ile of the native molecule. Similarly at position 34, only Phe could be effectively substituted for Tyr. Finally Q89A and Q89N completely abrogated the antibody binding.