MDCK cells were infected with six human influenza C virus strains (isolated between 1947 and 1981) and seven pig influenza C virus strains (isolated in 1981 and 1982) and the virus-specific polypeptides were compared by SDS-polyacrylamide gel electrophoresis and one-dimensional peptide mapping. The major structural polypeptides, i.e. glycoprotein (gp88), nucleoprotein (NP), and membrane protein (M), and one non-structural polypeptide were identified in all strains by radiolabelling infected cells with [35S]methionine. No differences in the electrophoretic migration of the M proteins or NS proteins were observed. The two earliest human isolates, C/Taylor/1233/47 and C/Great Lakes/1167/54, had faster migrating NP proteins, and another human strain, C/Georgia/1/69, displayed a faster migrating gp88. Minor differences in the one-dimensional peptide maps produced by partial digestion of the M proteins with V8 protease were observed between the human and pig isolates, while more marked differences were noted in the peptide maps of the glycoproteins of the C/Georgia/1/69, C/Yamagata/10/81 and C/Yamagata/11/81 viruses compared to the other human strains and the pig strains. The overall conclusion is that the proteins of human influenza C viruses isolated over a 35 year period and those of recent pig influenza C virus isolates are highly conserved.