The Arginase activity assay is a ready-to-use assay for the quantification of the enzymatic activity of Arginase in tissues samples. Arginase is an hydrolytic enzyme that converts arginine to ornithine and urea (fig 1). It is ubiquitous to all domains of life. The enzyme exists as two isoforms, arginase-I and arginase-II, which are encoded by different genes. Although arginase-I and arginase-II have similar enzymatic activities, they differ in isoelectric point (pI) and immunological reactivity. Human arginase-I (or liver-type arginase) is a 35 kD protein circulating in blood probably as a homotrimer. Liver-type arginase functions mainly in the urea cycle and is therefore abundantly expressed in mammalian liver. Arginase II is expressed in mitochondria of several tissues in the body, such as the kidney and prostate. It may also be found at lower levels in macrophages, lactating mammary glands, and brain tissue. 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.The Arginase activity assay provides an easy and fast procedure for measuring arginase activity in several sample preparations such as tissue- or cell lysates. Arginine solution (included as component is this kit) is incubated together with the samples of interest. The arginase present in these samples will convert arginine to urea and ornithine. The produced urea will react with the substrate ISPF (α-Isonitrosopropiophenone) thereby generating a colored product that can be measured and quantified. The assay has a detection limit of 0.35 units per mg protein.