A method is described for measuring (-)-threo-chlorocitric acid in human plasma. Plasma is acidified to pH 1 to minimize lactonization and a 13C analogue of (-)-threo-chlorocitric acid is added as internal standard. The acidified plasma is then extracted with ethyl acetate containing 10% methanol. The ethyl acetate-methanol extract is back-extracted with acetate buffer (pH 5). This extract, following adjustment to pH 1, is reextracted with ethyl acetate. The residue after removal of the ethyl acetate is treated with ethereal diazomethane. The wet residue is reconstituted in ethyl acetate and a portion of this solution is analyzed by gas chromatography-chemical ionization mass spectrometry. The mass spectrometer is set to monitor m/z 269 [MH+ of trimethylated (-)-threo-chlorocitric acid] and m/z 270 [MH+ of trimethylated (-)-threo-[13C]chlorocitric acid] in the gas chromatographic effluent. The m/z 269 to m/z 270 ion ratio in a sample containing an unknown amount of (-)-threo-chlorocitric acid is converted to an amount of compound using a calibration curve. The calibration curve is generated by analyzing control plasma spiked with various known amounts of (-)-threo-chlorocitric acid and a fixed amount of (-)-threo-[13C]chlorocitric acid. The limit of quantitation is 0.1-0.6 micrograms ml-1, depending on the characteristics of the calibration curve generated with each set of samples. The precision (relative standard deviation) at a concentration of 2 micrograms ml-1 is 3.3%.