The promotion of membrane fusion by the paramyxovirus hemagglutinin-neuraminidase (HN) and fusion (F) proteins requires that they be derived from homologous viruses, suggesting the possibility that the promotion of fusion requires a virus-specific communication between the two glycoprotein spikes. We have evaluated the ability of chimeric HN proteins, composed of domains from the HN proteins of two heterologous members of the group, human parainfluenza virus 3 (hPIV3) and Newcastle disease virus (NDV), to complement the F protein of each virus in the promotion of fusion. Specificity for the F protein of hPIV3 segregates with a segment composed of the transmembrane anchor and the first 82 residues of the ectodomain of its HN protein. Specificity of NDV HN for its homologous F protein is determined by a similar domain. These findings suggest that determinants specific to this segment of the attachment protein spike may be involved in the triggering of the fusion process.