Adenosine deaminase (adenosine aminohydrolase, EC 3.5.4.4; ADA) activity is widely distributed in human tissues and is highest in lymphoid tissues. Two ADA isozymes are known as ADA1 and ADA2. Human tissue extracts contained ADA1 predominantly. Meanwhile, ADA2 was the main component of serum ADA. ADA activity was significantly elevated in the sera from patients with hepatic diseases, hematological malignancies and infectious diseases. Serum concentrations of ADA1 were high in patients with acute leukemias, chronic myeloid blast crisis leukemia and acute liver injury. Serum ADA2 levels were raised in patients with adult T-cell leukemia, multiple myeloma (B-J type), infectious mononucleosis, rubella, acquired immunodeficiency syndrome, chronic hepatic diseases and tuberculosis. It is supposed that ADA1 is derived mainly from injured tissues or cells while ADA2 comes from stimulated T-cells.