A new advanced glycation end product (AGE), N-omega-carboxymethyl-arginine (CMA), was found in acid-soluble skin collagen of a newborn bovine prepared by in vitro glycation with 1 M glucose incubation at 37 degreesC for about 30 days [1]. CMA production was increased with incubation time in parallel, and after 30 days incubation the yield was 100 times higher than that of pentosidine [1]. This result suggested the importance of CMA as a major AGE in collagen. We have detected and measured the CMA level in human serum proteins by electrospray ionization/liquid chromatography/mass spectrometry (ESI/LC/MS), using CMA standard concentration curve. In this report, we first show the existence of CMA in vivo, and its serum level is significantly elevated in diabetic serum proteins, compared to age-matched control serum proteins. These results provide strong evidence that CMA is a new diagnostic marker of glycation in diabetes.