Egg-grown infectious bronchitis virus, strain Beaudette, was concentrated and centrifuged on sucrose density gradients to separate the virus into five peaks with densities of 1.144, 1.160, 1.172, 1.191 and 1.218 g/cm3. All peaks retained infectivity, complement fixation activity and were labelled with 3H-uridine. Morphologically the densest peak consisted of very large virus particles and amorphous material, the other peaks consisted of mainly intact particles although small differences in size and pleomorphism were seen. Polyacrylamide gel electrophoresis of material from the density gradient peaks revealed four major polypeptides and at least 10 minor polypeptides. The proportions of the polypeptides were approximately similar for all peaks with the exception of the densest peak in which the major polypeptides were greatly reduced. The four major polypeptides had approximate molecular weights of 1. 52,000, 2. 45,000, 3. 34,000, 4. 32,000. The major polypeptides 1 and 4 were shown to be glycosylated as were two of the minor polypeptides.