X-Prolyl dipeptidyl peptidase, which hydrolysed X-Pro-Y almost specifically, has been purified to homogeneity from crude cell-free extracts of Lactobacillus casei subsp. casei LLG using fast protein liquid chromatography equipped with preparative and analytical anion exchange columns. The enzyme was purified to 274-fold by ammonium sulphate fractionation, and by two successive ion-exchange chromatographies with a recovery of 34%. The purified enzyme appeared as a single band on both native-polyacrylamide gel electrophoresis (PAGE) and sodium dodecyl sulphate (SDS)-PAGE and had a molecular mass of 79 kDa. The pH and the temperature optima by the purified enzyme were 7.0 and 50 degrees C, respectively. X-PDP was a serine-dependent enzyme, as both diisopropylfluorophosphate and phenylmethylsulphonylfluoride caused complete inhibition of the enzyme activity. The Michaelis constant (K-m) and maximum reaction velocity (V-max) values were 0.2 mM and 43 mM per milligram, respectively.