An attempt was made to characterize the properties of adenosine diphosphate ribose pyrophosphohydrolase (ADPRase) in human tissues removed surgically from patients, which catalyzes the hydrolysis of ADP-ribose to AMP and ribose 5'-phosphate. The total activity was simillar in all tissues of palatine tonsil, the mucosa of maxillary sinus and inferior turbinate, but the distribution of its isoenzymes was tissue-specific. Mg²⁺-dependent form was mainly present in palatine tonsil, while Mg²⁺-independent one in the mucosa of maxillary sinus and inferior turbinate. In tonsillar tissue, Mg²⁺-dependent form of ADPRase was exclusively localized in the cytoplasmic fraction whereas, Mg²⁺-independet one distributed evenly in all fractions of cytoplasm, mitochondria and nuclei. Two isoenzymes seperated by ammonium sulfate fractionation from human tonsil were relatively stable without any loss of activity during the storage at 4℃ for several days, especially Mg²⁺-dependent form was more stable by maintaining its activity over 80% with the heat-treatment at 65°C for 10 minutes. The Mg²⁺-independent forms present in cytoplasm, mitochondria and nuclei showed similar inhibitory pattern by adenine nucleotides, suggesting that all Mg²⁺-independent forms in various subcellular fractions be the identical isoenzyme.