Paramyxovirinae envelope glycoproteins constitute a premier model to dissect how specific and dynamic interactions in multisubunit membrane protein complexes can control deep-seated conformational rearrangements. However, individual residues that determine reciprocal specificity of the viral attachment and fusion (F) proteins have not been identified. We have developed an assay based on a pair of canine distemper virus (CDV) F proteins (strains Onderstepoort (ODP) and Lederle) that share approximately 95% identity but differ in their ability to form functional complexes with the measles virus (MV) attachment protein (H). Characterization of CDV F chimeras and mutagenesis reveals four residues in CDV F-ODP (positions 164, 219, 233, and 317) required for productive interaction with MV H. Mutating these residues to the Lederle type disrupts triggering of F-ODP by MV H without affecting functionality when co-expressed with CDV H. Co-immunoprecipitation shows a stronger physical interaction of F-ODP than F-Lederle with MV H. Mutagenesis of MV F highlights the MV residues homologous to CDV F residues 233 and 317 as determinants for physical glycoprotein interaction and fusion activity under homotypic conditions. In assay reversal, the introduction of sections of the CDV H stalk into MV H shows a five-residue fragment (residues 110-114) to mediate specificity for CDV F-Lederle. All of the MV H stalk chimeras are surface-expressed, show hemadsorption activity, and trigger MV F. Combining the five-residue H chimera with the CDV F-ODP quadruple mutant partially restores activity, indicating that the residues identified in either glycoprotein contribute interdependently to the formation of functional complexes. Their localization in structural models of F and H suggests that placement in particular of F residue 233 in close proximity to the 110-114 region of H is structurally conceivable.