Docosahexaenoic acid (DHA; ) is an ω-3 fatty acid that is abundant in the brain and the retina and is known to be important in early development. Maresin 1 (7(R)-MaR1) is a 7,14-dihydroxy DHA formed from 14(S)-hydroperoxy DHA supplied exogenously to resident peritoneal mouse macrophages activated with zymosan A. It potently reduces infiltration of neutrophils into the mouse peritoneum in zymosan A-induced peritonitis when given intravenously (0.2 ng) and significantly increases phagocytosis of FITC-labeled zymosan A by isolated mouse macrophages (0.1-100 nM). Analytical and biological comparisons of synthetic 7(R)-MaR1 with endogenously derived 7(R)-MaR1 have confirmed its identity as matching the natural product.