Although smallpox has "been successfully eradicated, there is still some concern about the possibility of an animal reservoir of smallpox which might cause the disease to reappear. Many poxviruses which closely resemble smallpox in experimental studies have been isolated from mammals and these variola-like viruses have been investigated to determine their relationship to variola virus . Such comparisons have been made using biological characters, antigenic properties and biochemical analysis of the poxvirus genomes and virus induced proteins. In this thesis certain poxviruses have been compared by the extent to which the phenotypic expression of a virus determined mechanism occurs, resulting in the induction of enzymes involved with arginine biosynthesis, which is essential for poxvirus replication. Mammalian cells in vivo may synthesise arginine from citrulline via the Krebs-Kenseleit urea cycle utilising argininosuccinate synthetase/lyase activity. Many animal cells in vitro can .grow normally if citrulline replaces arginine but certain cell lines lack this capacity and exhibit an absolute requirement for arginine. The growth of one such line, mouse sarcoma 130 (S180) is inhibited in the presence of citrulline yet the replication of rabbitpox virus can., occur in these cells under such conditions. However when Sl80 cells are infected with cowpox virus and maintained in the presence of citrulline, no infectious progeny is produced. These results suggest that argininosuccinate synthetase/ lyase activit y is induced in rabbitpox virus infected cells b ut this character is not expressed during cowpox virus replication. In this thesis the replication of twelve different variola-like poxviruses (orthopoxviruses) was compared in the presence of citrulline in four cell types unable to utilise citrulline for cell growth. Two established cell lines, S130 and BHH 21 and two primary cultures derived from the chick embryo, cells from the chorioallantoic membrane and chick embryo fibroblasts were Lised. Virus yields were determined in all systems and further studies of virusspecific DTTA synthesis were made. The replication of all poxviruses examined was dependant on the availability of arginine• These poxviruses were found not to exhibit an absolute effect in the utilisation of citrulline but gave reduced yields relative to appropriate controls. These ranged from rabbitpox and vaccinia which produced high yields through to cowpox which gave a very low 77ield if citrulline replaced am i nine • Higher levels of virus-specific BTTA synthesis were found during rabbi tpozc vim s replication in the presence of citrulline compared with cownox virus replication under similar conditions. Certain viruses gave significant yields in the absence of exogenous arginine or citrulline and further studies were persued to ascertain the origin of the arginine needed for their replication. Some evidence indicated that an arginine pool was formed from increased protein turnover in the virus infected cells.