Eicosapentaenoic acid is metabolized, in part, through cytochrome P450-catalyzed epoxidation followed by conversion to the vicinal diols by epoxide hydrolases. (±)5(6)-DiHETE is a possible metabolite produced from EPA following epoxidation of the α-5 double bond. (±)5(6)-DiHETE lactone is a 1,5 cyclic ester derived from (±)5(6)-DiHETE. While its biological activity is unknown, the selective capacity of (±)5(6)-DiHETE to form this lactone can be utilized to specifically quantify (±)5(6)-DiHETE in biological samples.