An enzyme immunoassay for the quantitative measurement of proinsulin (intact) in serum and plasma. The measurement of Proinsulin in serum can provide valuable information for the diagnosis of insulinomas. Proinsulin levels have also been shown to be elevated in non-insulin dependent diabetics (NIDDM), newly diagnosed insulin dependent diabetics (IDDM) and other clinical situations. Proinsulin is a 9390 MW polypeptide of 86 amino acids, that is synthesized in the β cells of the pancreas and is the precursor molecule for insulin (1, 2, 3). Most proinsulin is converted to insulin and C-Peptide, which are secreted in equimolar amounts into the blood. About 15 % is not converted and is released as proinsulin. The biological activity of proinsulin is only about 10% of Insulin, but the half life of proinsulin is three times as long as insulin. The level of proinsulin in serum can be a reflection of β cell status. Both IDDM and NIDDM are characterized by dysfunction of the pancreatic β cells. Elevated proinsulin levels have been noted at the onset of IDDM and in healthy siblings of IDDM patients. Proinsulin levels may also be increased in patients with established NIDDM. Increased levels of circulating proinsulin are found in older patients, pregnant or obese diabetics, patients with insulinomas, functional hypoglycemia and hyperinsulinemia, a rare syndrome. Because the structure of proinsulin is similar to insulin, proinsulin may be detected as immunoreactive insulin in the insulin assay. Immunoreactive insulin levels are generally determined in conventional RIA's, which overestimate the insulin level because the methods use antibodies which crossreact with proinsulin. By calculating the molar ration of proinsulin to true insulin (P/I), a better assessment of β cell function can be made. The DRG Proinsulin ELISA Kit is a solid phase enzyme-linked immunosorbent assay (ELISA) based on the sandwich principle. The microtiter wells are coated with a monoclonal antibody directed towards a unique antigenic site on a Proinsulin molecule. An aliquot of patient sample containing endogenous Proinsulin is incubated in the coated well with enzyme conjugate, which is an anti- proinsulin antibody conjugated with horseradish peroxidase. After incubation the unbound conjugate is washed off. The amount of bound peroxidase is proportional to the concentration of Proinsulin in the sample. Having added the substrate solution, the intensity of colour developed is proportional to the concentration of Proinsulin in the patient sample.