The aim of this project was to study the stability characteristics of the serine protease dipeptidyl peptidase IV (DPP IV) and recombinant Fasciola hepatica cysteine protease cathepsin LI (rFheCLl), with a view to using them in peptide synthesis. DPP IV was purified from bovine serum. The chromatographic techniques used were phenyl-Sepharose hydrophobic interaction, Sephacryl S-300-HR gel filtration and quaternary-Sepharose anion exchange, producing an overall purification factor of 35. The enzyme was shown to have a subunit molecular weight of 77,224 Da, by SDSPAGE; native weight was not determined. DPP IV activity remained stable up to 62°C and over a broad pH range, with a preference for neutral pH. The estimated T5o was 71°C. With regard to solvent tolerance DPP IV was most stable in acetonitrile, at all concentrations tested. THF was the most deleterious solvent with little or no activity exhibited above 40% (v/v) concentration. Michaelis-Menten kinetics of DPP IV were determined at different pH values, pK values were observed at pH 6.5 and 9.5. The lower pK value is likely due to the ionisation of the active site histidine side chain of soluble DPP IV; however, the upper pK value possibly belongs to the a-amino group of the Gly moiety of the Gly-Pro-AMC substrate. rFheCLl was characterised and compared with the enzyme purified from liver fluke (Fasciola hepatica). rFheCLl was also stabilised by the reaction of the enzyme with ethylene glycol-bis(succinic acid N-hydroxy-succinimide ester) (EG-NHS). The enhanced stability of the modified enzyme is likely due to an intramolecular crosslink(s) being formed in the enzyme. The estimated T5o of modified rFheCLl was 87°C as compared to native rFheCLl, which had an estimated T50 of 78°C. At 70°C modified rFheCLl retained 100% amidase activity for up to 60 minutes compared to native rFheCLl, which showed an 80% loss in activity under the same assay conditions. Modified rFheCLl also demonstrated enhanced stability in the solvents acetone and acetonitrile; however, modification did not enhance stability in THF. Kinetic studies indicate that the active site of the enzyme has not been affected by cross-linking and that stability gain has not been at a cost of poorer catalysis. Native rFheCLl was used to catalyse the synthesis of the model tripeptide Z-Phe-Arg-SerNH2. LC-MS analysis indicated that formation of the new tripeptide occurred within ten minutes.