Arginase-I (liver-type Arginase, L-arginine ureahydrolase, or L-arginine amidinohydrolase) is an hydrolytic enzyme that converts arginine to ornithine and urea. Ornithine is the precursor for preline, an essential amino acid for collagen synthesis and for poly-amines, key components in cell growth and differentiation. Arginase exists as two forms designated Arginase-I and Arginase-II which are encoded by different genes. Although Arginase-I and Arginase-II have similar enzyme activities, they differ in immunological reactivity. Human Arginase-I (liver-type Arginase) is a 35 kDa protein circulating in blood probably as a homotrimer. Liver-type Arginase is most abundant expressed in mammalian liver, but is also found in non-hepatic tissues, for instance red blood cells, lactating mammalian glands, and the kidney. In addition to its involvement in ammonia detoxification via the urea cycle, Arginase plays a role in other processes, for instance macrophage-mediated cytotoxicity due to Arginase release and inhibition of lymphocyte proliferation in vitro. It shows high activity in growing tissues, wound healing, proliferating lymphocytes and tumors. Furthermore, Arginase acts as a modulator of the immune response. Besides this Arginase plays a role in allergen challenged lungs, in autoimmune inflammation in the central nervous system and in acute liver injury.