Aminoethylcysteine ketimine decarboxylated dimer (AECK-DD) is a natural sulfur-containing tricyclic compound detected, until now, in human urine and bovine cerebellum. Recently, the antioxidant properties of this compound, and particularly its protective effect on the in vitro oxidation of low-density lipoproteins, have been demonstrated. In this paper, the identification of AECK-DD in human plasma by means of gas chromatography, high-performance liquid chromatography and gas chromatography-mass spectrometry, performed after a simple and fast purification procedure, is reported.