Objectives: Amino acid levels in serum or plasma are used for early detection and diagnosis of several diseases. The objective of this study was to analyze amino acid levels in serum exosomes, which have not been previously reported.Design and methods: We investigated the amino acid composition of exosomes from human serum using HPLC with fluorescence detection.Results: The composition ratios of His, Arg, Glu, Cys-Cys, Lys, and Tyr were significantly increased in the exosomes compared with those in the corresponding native serum. D-Ser, an endogenous co-agonist of the N-methyl-D-aspartate receptor, was also enriched in the exosome-eluted fraction.Conclusions: Our results suggest that certain amino acids are enriched in the exosome-eluted fraction from human serum. These differences could have future diagnostic potential.