The work described in this thesis identifies some properties of the major polypeptides of pneumonia virus of mice (PVM) and of turkey rhinotracheitis (TRT) virus. The PVM glycoproteins have been studied in particular detail while the results obtained with TRT virus provide a preliminary description of the polypeptides of this virus. Twelve major PVM specific polypeptides designated L, G1, G2, F1, N, 39K, 35K, M, 20K, 19K, 16K and 12K were identified. In addition PVM specific polypeptides designated 25K, 24K, 23K, 18K and 17K were sometimes detected. Monoclonal antibodies directed against the G1/G2, 39K and M polypeptides were produced. The a~ility of a monoclonal antibody to precipitate G1 and G2 suggested that these two glycosylated proteins were related and this was confirmed by tryptic peptide mapping. G2 was shown to be derived from G1 in pulse chase experiments and a similar relationship between two higher mobility polypeptides synthesized in the presence of tunicamycin was observed. The G protein may have a precursor since G1 did not appear immediately following a pulse labelling. The precursor could not however be identified. An additional minor glycosylated polypeptide of 42K was found to be related to the G protein. The F1 protein appeared to be poorly glycosylated and a difference in mobility of the polypeptide synthesized in the presence of tunicamycin did not appear to be directly due to a lack of N-linked oligosaccharides. The polypeptide migrated more slowly under non-reducing conditions but no evidence of a small disulphide bonded polypeptide was found in contrast to the situation with other paramyxoviruses. This polypeptide appeared to be the major PVM protein expressed on the cell surface and was associated with G1 and G2 as the major protein in a particulate fraction of the infected cell supernatant. Tentative relationships were suggested between the 39K, 35K and 25K polypeptides, the M and 24K polypeptides and the 20K and 19K polypeptides. This together with the observation that the 12K polypeptide was not a primary gene product suggested that there may be about 11 PVM polypeptides. The N or 39K and the 20K or 19K polypeptides were observed to be phosphorylated. Twelve possible TRT virus specific polypeptides of 150K, 129K, 95K, 83K, 57K, 45K, 38K, 35K, 3DK, 23K, 19K and 15K were identified. The 150K, 95K, 83K, 57K, 45K and 15K polypeptides were glycosylated with the latter three polypeptides showing a similar relationship to the F1,2, F1 and F2 polypeptides of paramyxoviruses. A broad glycosylated band designated the 31K polypeptide was identified that was similar to a smeared band observed on prolonged exposure of immunoprecipitates of PVM polypeptides labelled with [3H]-glucosamine. The 35K and 19K polypeptides were observed to be phosphorylated. PVM may be more closely related to RS virus than TRT virus since anti-PVM serum irnmunoprecipitated the RS virus N polypeptide but not any TRT virus polypeptides. The PVM 39K polypeptide and the RS virus P protein were recognised by a monoclonal antibody providing further evidence of a relationship between PVM and RS virus.