The hemagglutinin-neuraminidase (HN) glycoprotein of Newcastle disease virus was isolated by cleaving HN (cHN) from reconstituted virosome with chymotrypsin. N-terminal sequence analysis of the purified cHN showed that chymotrypsin cleavage had occurred at amino acid 123, freeing the C-terminal 454 amino acids. The purified cHN retained its neuraminidase and receptor binding activities and reacted with specific monoclonal antibodies, showing that the isolated cHN was biologically and antigenically functional. The crystals of the cHN were obtained in acetate buffer (pH 4.6) containing polyethylene glycol 3350 and ammonium sulfate and belong to the orthorhombic space group P2(1)2(1)2(1) with unit cell dimension of approximately a = 72 A, b = 78 A, and c = 198 A. Crystals of cHN grown in the presence of sialic acid (Neu5Ac) were grown in HEPES buffer (pH 6.2) containing polyethylene glycol 3350 and belong to the hexagonal space groups P6(1) or P6(5) with unit cell dimensions of a = b = 137.5 A and c = 116.6A. The orthorhombic crystals produced in this study diffract X rays to at least 2.0-A resolution, thereby setting the stage for the solution of the three-dimensional structure of the HN glycoprotein of a paramyxovirus. Copyright 2000 Academic Press.