An acetylated derivative of β-endorphin; binds to opioid receptors in rat caudal dorsomedial medulla (Ki = 490 nM) and mouse brain membrane preparations (IC50 = 109 nM); induces chemotaxis of human monocytes in vitro (0.1 nM); reduces β-endorphin-, morphine-, DAMGO-, and DADLE-induced antinociception in mice in a tail-flick test (ED50s = 2.08, 33.11, 0.03, and 0.07 mol per animal., respectively)