We have previously identified five antigenic sites on the hemagglutinin-neuraminidase (HN) glycoprotein of the Australia-Victoria isolate of Newcastle disease virus (Iorio and Bratt, J. Virol. 48, 440-450; Iorio et al., J. Gen. Virol. 67, 1393-1403). Two additional sites (designated 12 and 23) are now described, bringing to a total of seven the number of antigenic sites defined by our panel of neutralizing anti-HN antibodies. Competition antibody binding and additive neutralization assays reveal that each of these newly-identified sites overlaps two previously-defined ones. The seven HN antigenic sites thus form a continuum in the three-dimensional conformation of the molecule. Studies on the inhibition of hemagglutination (HA), neuraminidase (NA) and the attachment of virus to chick cell monolayers have been used to construct a functional profile of each antigenic site. Monoclonal antibodies (mAbs) to three overlapping sites (12, 2 and 23) inhibit HA and NA and prevent viral attachment to chick cell monolayers. These findings are consistent with the domains recognized by these mAbs being close to the NA and receptor-binding sites. MAbs to two other overlapping sites, 14 and 1 (which in turn, overlap site 12), inhibit HA quite effectively, and attachment to a lesser extent. Sites 14 and 1 probably identify a second domain involved in receptor recognition. MAbs to the two remaining sites (3 and 4), though neutralizing, are negative in all three assays, thus recognizing domains not involved in HA or NA or attachment to chick cells.