Aminomalonic acid (Ama) was first detected in alkaline hydrolysates of proteins in 1984. In this work we describe our search for the origin of aminomalonic acid in alkaline hydrolysates of proteins. We have developed a technique for quantitation of aminomalonic acid based upon gas chromatography/mass spectrometry. Using this technique, we find approximately 0.3 Ama/1000 amino acids in hydrolysates of Escherichia coli protein. We have demonstrated that Ama is not formed from any of the 20 major amino acids during the hydrolysis procedure. Furthermore, the amount of Ama found does not depend on the presence of small amounts of O2 during the hydrolysis. Thus far, we have not been able to demonstrate an artifactual origin for Ama. The results described above suggest that Ama may indeed be a constituent of proteins before the hydrolysis procedure. Possible origins of Ama include errors in protein synthesis and oxidative damage to amino acid residues in proteins.