Analysis of differentially radiolabelled avian infectious bronchitis virus (IBV) indicated that the matrix (M) polypeptides of mol. wt. 23 X 10(3) (23K), 26K, 28K, 30K and 34K (M23 to M34) which have been shown to give the same peptide maps, differed in their degree of glycosylation; M23 was not glycosylated while glycosylation increased with increasing mol. wt. from M26 to M34. Both glucosamine and mannose were components of M26 to M34 but [3H]fucose appeared to be associated mainly with M34. Endo-beta-N-acetylglucosaminidase H removed oligosaccharides from M28 and M30 but not M26 and M34, to give a polypeptide of 23K. The surface projection glycopolypeptides S1 (90K) and S2 (84K) incorporated 3H-labelled glucosamine and mannose but not fucose and had oligosaccharides removed by endoglycosidase H. The mol. wt. of the resultant polypeptides varied among experiments; the lowest mol. wt. observed were 64K and 61K. These results indicate (i) that the polypeptide moieties of the S polypeptides are approximately 64K and 61K, and 23K for the M polypeptide, (ii) that the oligosaccharides of the S and M polypeptides are of the high-mannose type and are linked to the polypeptides by N-glycosidic linkages, and (iii) that the M glycoprotein of IBV differs from that of murine coronaviruses and bovine coronavirus L9 which have O-linked oligosaccharides.