A simplified procedure for the purification of adenosine deaminase(ADA)from human thymus based on immuno-affinity chromatography of anti-human thymus ADA IgG was described. After ammonium sulfate fractionation, immuno-affinity chromatography and Sephadex G-100 gel filtration. ADA was separated as homogeneity from human thymus.The yield and the specific activity of purified ADA were 34.15% and 14898 U/mg respectively. The purified ADA molecule consists of about 380 amino acid residues giving a Mr of 41.3 ku and pI of 4.9. The optimum temperature is 37～40℃. The optimum pH is 7.0. Using adenosine or 2-deoxyadenosine as substrate the apparent K_m of the enzyme is 83 μmol/ L and 61 μmol/ L respectively. The enzyme activity can be inhibited by p-chloromercuric benzoic acid while partially restored by dithiothreitol. ADA activity was decreased by anti-calf ADA IgG and anti-thymus ADA IgG.