The virion polypeptide composition of three independently isolated tree shrew herpesviruses (THV) was analysed by SDS-polyacrylamide slab gel electrophoresis and by a two-dimensional technique using isoelectric focusing. Two of the virus isolates analysed were from malignant tumours; the other isolate (THV, strain 1) was from an apparently healthy animal. The polypeptide patterns of the three purified Tupaia herpesvirus isolates were remarkably similar, each consisting of at least 35 polypeptides ranging in mol. wt. from 12,000 to 230,000. Whilst the majority of analogous polypeptides of the three viruses were of indistinguishable electrophoretic mobility, some (e.g. polypeptides of 82K to 86K) showed small differences in apparent mol. wt. which were characteristic of the virus strain. Comparative SDS-polyacrylamide gel electrophoresis made it possible to distinguish the Tupaia herpesvirus isolates from each other. At least five glycoproteins were found in purified THV virions. The two-dimensional electropherograms revealed at least 47 discernible protein spots, some of which were specific for a given THV isolate and which were detectable even in lysates of THV-infected cells.