The association of folates with the prevention of neural tube defects and reduced risk of other chronic diseases has stimulated interest in the development of techniques for the study of their bioavailability in humans. Stable isotope protocols differentiate between oral and/or intravenous test doses of folate and natural levels of folate already present in the body. An liquid chromatography/mass spectrometry (LC/MS) procedure is described that has been validated for the determination of [ 13 C]5-methyltetrahydropteroyl monoglutamic acid ([ 13 C]5-CH 3 H 4 PteGlu) in plasma and urine, following oral dosing of volunteers with different labeled folates. Folate binding protein affinity columns were used for sample purification prior to LC/MS determination. Chromatographic separation was achieved using a Superspher 100RP18 (4 m) column and mobile phase of 0.1 mol/L acetic acid (pH 3.3):acetonitrile (90:10; 250 L/min). Selected ion monitoring was conducted on the [M-H] Ψ ion: m/z 458 and 459 for analyzing 5-CH 3 H 4 PteGlu; m/z 464 [MΨ6-H] Ψ to determine 5-CH 3 H 4 PteGlu derived from the label dose; m/z 444 for analysis of 2 H 4 PteGlu internal standard, and m/z 446 and 478 to confirm that there was no direct absorption of unmetabolized compounds. Calibration was linear over the range 0 -9 Ψ 10 Ψ9 mol/L; the limits of detection and quantification were 0.2 Ψ 10 Ψ9 and 0.55 Ψ 10 Ψ9 mol/L, respectively. The mean coefficient of variation of the ratios (m/z 463/458) was 7.4%. The method has potential applications for other key folates involved in one-carbon metabolism.