The roles of N-linked glycosylation in the intracellular transport and biological activities of the Sendai virus hemagglutinin-neuraminidase (HN) protein were studied. The protein contains four potential N-glycosylation sites: N77, N448, N499, and N511. By site-directed mutagenesis of these positions, the mature protein contained three N-linked oligosaccharides attached to N77, N499, and N511. The role of each added oligosaccharide in the structure and functions of the protein was identified by characterization of surface expression, hemadsorption, and neuraminidase activities of the corresponding mutant proteins. Elimination of the sites of N499 and N511 had the most detrimental effect, decreasing surface expression and hemadsorption. Elimination of the sites of N77 and N448 had similar but weaker effects. Mutants missing the sites of N499 and N511 were not able to induce syncytia formation in cells expressing mutant HN proteins and the fusion protein. Therefore, the N-linked oligosaccharides attached to N499 and N511 were important for intracellular transport and for the fusion promotion.