Oxalate oxidase is a water soluble, thermolabile, homo-oligomeric glycoprotein the synthesis of which marks the onset of germination In wheat and barley embryos. The protein Is also highly abundant In barley roots. The enzyme has an average oligomer molecular mass of about 115 kDa and about 22.8 kDa for the monomers, as determined by mass spectrometry. The ollgomeric cereal oxalate oxidases are resistant to dissociation In SDS containing media and to digestion by pepsin. The cereal organs produce two oxalate oxidase Isoforms (G and G') which possess the same apoprotein but are differentially glycosylated. The oligosaccharide side chain(s) has a molecular mass of about 2-3 kDa. Barley root also contains a third active oxalate oxidase isoform with a mass of about 22.5 kDa, which was not detected in germinating embryos of the same cultlvar. All of the cereal oxalate oxidases were shown to have identical N-terminal amino acid sequences and almost identical kinetic properties This thesis describes the characterisation of oxalate oxidases Isolated from three transgenic plants lines, expressing chimeric CaMV 35S-oxalate oxidase genes. SGS5 tobacco was expressing a gene with the native oxalate oxidase signal peptide and 3S1 oilseed rape and C26 tobacco were expressing a gene containing a foreign extensin signal peptide. Transgenic SGS5 tobacco produced an oxalate oxidase which was almost indistinguishable from the native cereal protein, in terms of Its structure, stability, enzyme activity and resistance to dissociation In SDS containing media and digestion by pepsin. This work Illustrated the ability of a dicotyledonous plant (tobacco) to recognised and correctly process a transgenic monocotyledon protein (wheat).Transgenic 3S1 oilseed rape and C26 tobacco were shown to produce active oligomeric oxalate oxidases, which did not exhibit any of the unusual resistance properties normally associated with these proteins. Instead the 3S1 and C26 oxalate oxidases were unstable and exhibited significantly altered kinetic properties compared with the native cereal and transgenic SGS5 enzymes. The instability was thought to have arisen from the Incorrect processing of the 3S1 and C26 oxalate oxidases, resulting in the partial cleavage of the extensin signal peptide, which in turn gave rise to a mature oxalate oxidase with an altered N- terminal sequence compared with the native cereal enzyme. The use of vacuum infiltration confirmed the association of the transgenic enzymes with the extracellular spaces, although the majority of the enzyme was shown to be intracellular. The main objective for producing the transgenic oilseed rape expressing oxalate oxidase was to Improve fungal pathogen resistance against oxalic acid secreting pathogens. The results described in this thesis are concerned with a direct comparison of the structure, stability and kinetics between the native cereal and transgenic oxalate oxidases and the possible consequences for pathogen resistance In plants expressing unstable yet active transgenic enzymes.