The human respiratory coronavirus OC43 was grown on a human rectal tumor cell line and was isotopically labeled with amino acids, glucosamine, and orthophosphate to analyze virion structural proteins. Four major protein species were resolved by electrophoresis and many of their properties were deduced from digestion studies using proteolytic enzymes. The four proteins are: A 190 kDa protein, the presumed peplomeric protein, that was glycosylated and proteolytically cleavable by trypsin into subunits of 110 and 90 kDa. The subunits each represent a different amino acid sequence on the basis of peptide mapping; a 130 kDa protein that was glycosylated and behaved as a disulfide-linked dimer of 65 kDa molecules. It is the apparent virion hemagglutinin on the basis of digestion studies with trypsin, bromelain and pronase; a 55 kDa nucleocapsid protein that was phosphorylated; a 26 kDa matrix protein that was glycosylated. The 190, 130, 55 and 26 kDa species can therefore be designated P, H, N and M, respectively. They exist in molar ratios of 4:1:33:33, and are calculated to be present at the rate of 88, 22, 726, and 726 molecules per virion, respectively.