Arginase is a divalent cation-dependent enzyme that converts L-arginine into nonprotein amino acid L-ornithine and urea; it is the final enzyme of the urea cycle. Arginase is a trimeric metalloenzyme and Mn2+ is the physiologic activator. Mammals have two arginase isozymes, arginase I and arginase II, which are encoded by different genes. Arginase I, also called liver arginase, is highly expressed in the cytosol of hepatocytes and is involved in ammonia detoxification in the liver. Arginase II is localized in the mitochondria in extrahepatic tissues, such as kidney, brain, prostate, small intestine, lactating mammary gland, and skeletal muscle, and provides L-ornithine for proline and polyamine biosynthesis. Since L-arginine is also the substrate for nitric oxide (NO) synthase (NOS), it has been shown that both arginase isozymes can reciprocally regulate NOS activity by modulating L-arginine availability. Therefore, arginase I and II modulate many biological functions through regulating NO production. The role of arginase I in the immune response, including anti-parasite and anti-viral responses, has been studied. The induction of mouse (but not human) arginase I in macrophages by exogenous stimuli, including Th2 cytokines IL-4, IL-10, and IL-13, is well known. A study has shown that alternative activated monocytes/macrophages from humans affected by filariasis can express arginase I. Arginase I is constitutively expressed in human granulocytes and is localized to the azurophil granules, and is involved in antimicrobial activity. The role of arginase I in tumor biology has also been studied; in this sense, arginase I may promote growth of some types of tumors via polyamine synthesis or downregulation of NO-mediated tumor cytotoxicity. In contrast, arginase I can also promote death of some types of tumors by L-arginine depletion. The involvement of arginase I in axonal regeneration in the central nervous system after injury has also been reported.