Octanoic Acid-13C is intended for use as an internal standard for the quantification of octanoic acid by GC- or LC-MS. Octanoic acid is a medium-chain saturated fatty acid. It has been found in Teleme cheeses made from goat,ovine,or bovine milk.1 Octanoic acid is active against the bacteria S. mutans,S. gordonii,F. nucleatum,and P. gingivalis (IC80s = <125,<125,1,403,and 2,294 μM,respectively).2 Levels of octanoic acid are increased in the plasma of patients with medium-chain acyl-CoA dehydrogenase (MCAD)deficiency,an inborn error of fatty acid metabolism characterized by hypoketotic hypoglycemia,medium-chain dicarboxylic aciduria,and intolerance to fasting.3,4