The precursor of fusion protein (F0) in Sendai virus grown in Vero [African green monkey kidney] cells can be cleaved by trypsin to forms F1 and F2, which can be resolved on SDS[sodium dodecyl sulfate]-polyacrylamide gels. If disulfide bonds are preserved during electrophoresis, F1 and F2 remain linked together even after trypsin treatment (F). Sendai virus grown in embryonated chicken eggs does not contain the precursor F0. An F protein was found for Sendai virus grown in eggs when disulfide bonds were preserved during electrophoresis. The hemagglutinin-neuraminidase (HN) glycoproteins also appear to be disulfide-linked to form large complexes which are observed on SDS-polyacrylamide gels of nonreduced samples.