Advanced glycation end products (AGEs) derived from glucose are implicated in the pathogenesis of diabetic vascular disease. However, many lines of evidence suggest that other pathways also promote AGE formation. One potential mechanism involves oxidants produced by the NADPH oxidase of neutrophils, monocytes, and macrophages. In vitro studies have demonstrated that glycolaldehyde, a product of serine oxidation, reacts with proteins to form N^ε-(carboxymethyl)lysine (CML), a chemically well-characterized AGE. We used mice deficient in phagocyte NADPH oxidase (gp91-phox^−/−) to explore the role of oxidants in AGE production in isolated neutrophils and intact animals. Activated neutrophils harvested from wild-type mice generated CML on ribonuclease A (RNase A), a model protein, by a pathway that required l-serine. CML formation by gp91-phox^−/− neutrophils was impaired, suggesting that oxidants produced by phagocyte NADPH oxidase contribute to the cellular formation of AGEs. To determine whether these observations are physiologically relevant, we used isotope-dilution gas chromatography/mass spectrometry to quantify levels of protein-bound CML in mice suffering from acute peritoneal inflammation. Phagocytes from the gp91-phox^−/− mice contained much lower levels of CML than those from the wild-type mice. Therefore, oxidants generated by phagocyte NADPH oxidase may play a role in AGE formation in vivo by a glucose-independent pathway.