Protectin D1-d5 is intended for use as an internal standard for the quantification of protectin D1 by GC- or LC-MS. Protectin D1 is a specialized pro-resolving mediator (SPM) synthesized from docosahexaenoic acid (DHA).¹ DHA is oxidized to 16S,17S-epoxy protectin, which is converted to protectin D1 enzymatically. Protectin D1 increases phagocytosis of apoptotic polymorphonuclear leukocytes (PMNs) by macrophages in a non-phlogistic manner and is generated in vitro during macrophage-apoptotic interactions.² It enhances phagocytosis in mice after 24 hours, but not at the initiation or peak of inflammation. It also decreases PMN infiltration in a zymosan-induced mouse model of inflammation when administered at a dose of 300 ng per animal. Protectin D1 (200 µg, i.v.) inhibits increases in neutrophil counts in bronchoalveolar fluid (BALF) and lung myeloperoxidase activity in a mouse model of pulmonary injury and inflammation induced by intratracheal LPS instillation.³ It also decreases pulmonary edema and promotes neutrophil apoptosis in BALF.