The adenosine deaminase from calf intestinal mucosa was modified with 2,4-bis(o-methoxy-polyethylene glycol)-6-chloro-s-triazine(activated PEG), which was synthesized from monomethoxypolyethylene glycol(MW 5,000) and cyanuric chloride. Approximately 41 out of the total 69 amino groups in the adenosine deaminase molecule were modified. The conjugation of PEG increased affinity for adenosine showed by decreased K_m. ADA and PEG-ADA had temperature optima of 60℃ and 50℃, respectively. Thermal inactivation at temperature above optima was essentially the same for the native and modified enzyme. PEG-ADA and ADA demonstrated an resistance to proteolytic digestion of trypsin and chymotrypsin. Modification of the adenosine deaminase with PEG resulted in greatly decreased electrophoretic mobility of the protein. The modified ADA preparations were less immunogenic and antigenic than native enzyme as determined by immunodiffusion studies.