A cDNA library was constructed in λgt10 using mRNA purified from cells infected with parainfluenza virus type 2 (PIV2). Virus-specific clones were identified by screening the library with 32P-labelled cDNA probes made from randomly primed vRNA. Clones containing the haemagglutinin-neuraminidase (HN) gene were identified by sequence comparisons with known parainfluenza virus HN gene sequences. The largest HN clone isolated had a nucleic acid sequence of 2065 bp with a single long open reading frame encoding a protein of 571 amino acids. The HN protein has nine predicted glycosylation sites and an amino-terminal membrane-spanning region. The PIV2 HN protein shares 43% amino acid identity with the HN protein of simian virus 5 and 40% with mumps virus, 30% of the amino acids being common to all three viruses.