The Massachusetts strain of avian infectious bronchitis virus was purified from embryonated hens' eggs by polyethylene glycol 6,000 precipitation and isopycnic centrifugation through Metrizamide gradients. Four major polypeptides of apparent molecular weight 90,000, 52,000, 29,000 and 26,000 were resolved by SDS-polyacrylamide gel electrophoresis. Omission of reducing agent failed to resolve the 29,000 molecular weight component and increased the mobility of the 90,000 molecular weight polypeptide. Labelling of acrylamide gels with 125 I-concanavalin A indicated that polypeptides of molecular weignt 90,000, 29,000 and 26,000 were glycosylated, and in the absence of reducing agent that the 29,000 glycopeptide migrated as a dimer in the 50,000 molecular weight region. Purified IBV radioiodinated with Bolton and Hunter reagent, which banded as a single peak of radioactivity in Metrizamide gradients, was found to contain bands of radioactivity when analysed by SDS-PAGE corresponding to the polypeptides of molecular weight 90,000, 52,000 and 29.000 resolved in stained gels. Disruption of IBV particles in Triton X-100 released two subviral particles; a 16 nm spike which was comprised of polypeptides with molecular weights of 90,000, 52.000 and 29,000; and another denser spherical particle 25-45 mm in diameter which contained nucleic acid and the 52,000 and 26,000 polypeptides. A model for the structure of IBV is presented.