We report a new method for the quantitative determination of human methemoglobin (metHb) based on the measurement of the solvent-water proton-nuclear magnetic resonance (NMR) relaxation rate R1 [normalized to 1 mmol/L hemoglobin (Hb) concentration]. MetHb (%) is estimated from the linear dependence of R1 on the metHb concentration, taking into account the simple relationship [MetHb] = [(R1 - R1HbO2)/(R1metHb - R1HbO2)].100, where R1HbO2 and R1metHb are values for the solvent-water relaxation rate of standard 1.0 mmol/L solutions of the oxygenated derivative of human hemoglobin (HbO2) and of metHb, respectively. The minimum metHb that may be determined from the analysis of the experimental data is 0.5 +/- 0.4%.