The quaternary structure of respiratory syncytial virus (RSV) fusion protein has been studied. Crosslinking studies were done to stabilize the noncovalently associated proteins. These stable, heat-resistant, covalently linked complexes were analyzed by sodium dodecyl sulfate-polyacrylamide electrophoresis. In situ crosslinking studies demonstrated that the fusion protein of RSV exists as a dimer in its native form on the surface of infected cells. The purified protein was also found to be present predominantly as a dimer. In addition, the results suggest that F1 subunits may play a role in the dimerization of the fusion protein.