Opa-expressing phenotypes of two Nm strains (C751 and MC58) were used to investigate their binding to the modified NA1B-Fc receptors. Three C751 derivatives express-ing distinct Opa proteins (OpaA, OpaB and OpaD) and the MC58 derivative expressing an Opa protein designated OpaX (Virji et al., 1999) were assessed by receptor overlay experiments (Fig. 3) .