The folding and oligomerization of coronavirus spike protein were explored using a panel of monoclonal antibodies. Chemical cross-linking and sedimentation experiments showed that the spike of transmissible gastroenteritis virus is a homotrimer of the S membrane glycoprotein. The spike protein was synthesized as a 175,000-apparent-molecular-weight (175K) monomer subunit that is sensitive to endo-beta-N-acetylglucosaminidase H. Assembly of monomers into a trimeric structure was found to occur on a partially trimmed polypeptide and to be a rate-limiting step, since large amounts of monomers failed to trimerize 1 h after completion of synthesis. Terminal glycosylation of newly assembled trimers, resulting in the biosynthesis of three 220K oligomers, occurred with a half time of approximately 20 min. Monomeric (230K to 240K) processed forms were also observed in cells and in virions. The 175K monomeric form expressed four major antigenic sites previously localized within the amino-terminal half of the S polypeptide chain; however, two classes of trimer-restricted epitopes (borne by three 220K and/or three 175K oligomers) were identified. The S glycoprotein of coronavirus might be a valuable model system for discovering new aspects of the maturation of membrane glycoproteins.