A novel glucose dehydrogenase (GDH) which reacts with 1,5-anhydro-D-glucitol (1,5AG), a clinical marker of diabetes, was purified from a soluble fraction of a newly isolated marine Gram-negative bacterium designated strain alpha-15, which was identified as a Deleya sp. This GDH is composed of a single peptide with a molecular weight of 67 kDa by SDS-PAGE and gel filtration chromatography. It cannot oxidize the C-1 hydroxy group of glucose which is commonly oxidized by other known GDHs or glucose oxidase.Since this enzyme reacts with 1,5AG, a navel enzymatic method for 1,5AG determination was developed utilizing the fluorescent electron mediator, AlamarBlue(TM). A good linear correlation was observed from 2.5-200 mu M of 1,5AG, which covers both the healthy subjects＇ and the diabetics＇ range in blood.