The role of intramolecular disulfide bonds in the fusion (F) protein of Sendai virus was studied. The 10 cysteine residues were changed to serine residues using site-directed mutagenesis. None of the cysteine mutant F proteins reacted with a monoclonal antibody specific for the mature conformation of the F protein, but eight of ten mutants reacted with an immature conformation-specific monoclonal antibody. The transport of these mutant proteins to the cell surface was drastically reduced. All of the cysteine mutant F proteins remained sensitive to endoglycosidase H (endo H) for 3 h after their synthesis. Moreover, cell surface transport of the hemagglutinin-neuraminidase (HN) protein co-expressed with each of these cysteine mutant F proteins was also reduced. These results suggest that all cysteine residues participate in the formation of intramolecular disulfide bonds, that co-translational disulfide bond formation is crucial to the correct folding and intracellular transport of the F protein, and that interaction of the F and HN proteins takes place intracellulary.