The binding of methylmercury, CH3Hg(II), by small molecules in the intracellular region of human erythrocytes has been studied by 1H-NMR spectroscopy. To suppress or completely eliminate interfering resonances from the much more abundant hemoglobin protons, spectra were measured by a technique based on the transfer of saturation throughout the envelope of hemoglobin resonances following a selective presaturation pulse or by the spin-echo Fourier transform method. With these techniques, 1H-NMR spectra were measured for the more abundant intracellular small molecules, including glycine, alanine, creatine, lactic acid, ergothioneine and glutathione, both intact and hemolyzed erythrocytes to which CH3Hg(II) had been added. The results for intact erythrocytes indicate that part of the CH3Hg(II) is complexed by intracellular glutathione. These results also indicate that exchange of CH3Hg(II) among glutathione molecules is fast, with the average lifetime of a CH3Hg(II)-glutathione complex estimated to be less than 0.01 s. From exchange-averaged chemical shifts of the resonance for the proton on the alpha-carbon of the cysteine residue of glutathione, it is shown that, in hemolyzed erythrocytes, the sulfhydryl group of glutathione binds CH3Hg(II) more strongly than the sulfhydryl groups of hemoglobin.