Resolvin D1-d5 (RvD1-d5) is intended for use as an internal standard for the quantification of RvD1 by GC- or LC-mass spectrometry. Resolvins are a family of potent lipid mediators derived from both eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).¹ In addition to being anti-inflammatory, resolvins promote the resolution of the inflammatory response back to a non-inflamed state.² RvD1 is produced physiologically from the sequential oxygenation of DHA by 15- and 5-lipoxygenase.¹ A 17(R)-epimer of RvD1 can also be generated in aspirin-treated samples.³ Both RvD1 and its 17(R) configuration reduce human polymorphonuclear leukocyte (PMNL) transendothelial migration, the earliest event in acute inflammation, with EC50 values of ~30 nM.⁴ RvD1 and its aspirin-triggered form also exhibit a dose-dependent reduction in leukocyte infiltration in a mouse model of peritonitis with a maximal inhibition of ~35% at a 10-100 ng dose.⁴