The GC-C-IRMS technique represents a new and useful tool to study the15N abundance in amino acids for metabolic studies. Here we present15N analysis data for 13 human plasma protein amino acids at natural abundance and in slightly enriched samples using GC-C-IRMS analysis of N-pivaloyl-i-propyl esters of amino acids. A mean precision of ± 0.71‰ Δ15N or ±0.77‰ Δ15N was determined for all amino acids analysed at natural abundance values and for the slightly enriched samples (< 100‰ δ15N), respectively. However, for individual amino acids at natural abundance derivatized and injected in duplicate the standard deviation ranged from 0.24 to 1.49‰. The lowest natural15N abundance was found for threonine (about - 5‰ δ15N) and the highest for proline and glutamic acid (about 15‰ δ15N). A standard deviation (SD) range of±0.71 (glutamic acid) to ± 4.35 ‰ δ15N (phenylalanine) (mean SD = ± 1.at δ15N) was obtained in protein amino acids in a group of 7 volunteers at natural abundance level. A single oral [15N2]urea bolus was administreted to one healthy volunteer to demonstrate the feasibility of the GC-C-IRMS technique to trace human nitrogen kinetics in plasma protein amino acids. Small but significant differences in15N abundance were found between amino acids, presumably due to their differential participation in transamination reactions.