The fusion (F) protein of mumps virus was purified by immunoaffinity chromatography using an anti-F monoclonal antibody. The F protein was reduced and alkylated, and the F1 and F2 chains were isolated by high-pressure size exclusion chromatography. Twenty-three amino acid residues from the amino terminus of each chain were identified following automated Edman degradation. The amino-terminal sequence of the F1 chain was homologous to previously reported F1 sequences from three other paramyxoviruses (simian virus 5, Newcastle disease virus, and Sendai virus). Secondary structure predictions suggest an a-helical conformation for the mumps virus F1 amino-terminal sequence. A helical wheel model of the paramyxovirus F1 NH2 terminus is presented which defines conserved and variable arcs of the helix and provides a spatial representation of this critical functional domain of the paramyxovirus fusion protein. © 1985.