A ninhydrin-positive compound was detected in acid hydrolysates of various alkali-treated bakery products (pretzels, snack bars), eluting immediately after pyridosine in amino acid chromatograms. Following preparative isolation from a food sample and independent synthesis, the compound was unequivocally identified by fast-atom bombardement-mass spectrometry, H-1- and C-13-nuclear magnetic resonance as a protein-bound imidazolone, existing in two tautomeric forms, namely N(delta)-(5-methyl-4-oxo-5-hydroimidazol-2-yl)-L-ornithine and N(delta)-(4-methyl-5-oxo-4-hydroimidazol-2-yl)-L-ornithine. The acid-labile amino acid derivative is formed by direct condensation of the guanido group of arginine and methylglyoxal, a sugar degradation product, and represents a previously unknown post-translational protein modification. For a number of commercially available alkali-treated bakery products, the amounts of the imidazolone after complete enzymic digestion ranged between 900 and 1300 mg per 100 g protein, indicating that between 20 and 30% of the arginyl residues might react with methylglyoxal during the bakery process.