Specific determination of N-(2-furoylrnethyl)valine (FM-Val) together with furosine in acid hydrolyzates of human hemoglobin of healthy volunteers (n = 6) and diabetic patients (n = 14) by means of reversed-phase HPLC with electrospray ionization-time-of-flight mass spectroscopy is reported. Whereas FM-Val is formed during acid hydrolysis of the N-terminal hemoglobin adduct N-fructosylvaline, furosine results from acid degradation of lysine residues glycated at the epsilon-amino group. Quantification was based on the use of synthesized isotopomers, namely N- [2-(C-13(6)) furoylmethyl]valine and N-epsilon-[2-(C-13(6))furoylmethyl] lysine, thus enabling interference-free detection and calibration. Taking the conversion factors into account, the amount of N-terminally bound N-fructosylvaline in human hemoglobin was between 518 and 774 pmol/mg protein for healthy volunteers and between 586 and 1426 pmol/mg protein for diabetic patients. Derivatization at the side chain of peptide-bound lysine residues to N-epsilon-fructosyllysine was from 1156 to 1753 pmol/mg protein for healthy controls and from 1191 to 2409 pmol/mg protein for diabetics. For these patients, the amount of N-fructosylvaline showed good correlation with the values for HbA(1c). The significantly higher relative extent of glycation at the N terminus compared to side-chain glycation points to a specific and intraindividual capacity for enzymatic deglycation in human erythrocytes, which can be assessed using the proposed method.