We recently demonstrated that early administration of rat adrenomedullin (AM), a vasoactive peptide, in combination with its binding protein (human AMBP-1) produces various beneficial effects in sepsis. Human AM is a 52-amino acid peptide, but rat AM differs from human AM, having only 50 amino acid residues, with two amino acid deletions and six substitutions. It remains unknown whether a combination of human AM and human AMBP-1 (AM/AMBP-1) is also beneficial in sepsis and, if so, whether human AM/AMBP-1 reverses established sepsis in rats. To test the effects of human AM/AMBP-1, we induced sepsis in male adult rats by cecal ligation and puncture (CLP). At 10 h after CLP (i.e., severe sepsis), human AM (12-48 microg/kg body weight) was administered in combination with human AMBP-1 (40-160 microg/kg body weight). Vehicle-treated animals received a nonspecific human plasma protein (albumin). Blood and intestinal samples were collected at 20 h for various measurements. In additional groups of septic animals, the gangrenous cecum was surgically excised at 20 h after CLP. The 10-day survival was recorded. Our results showed that tissue injury, as evidenced by increased levels of transaminases and lactate, was present at 20 h after CLP. Proinflammatory cytokines tumor necrosis factor-alpha and interleukin-6 were significantly elevated. Gut barrier dysfunction, manifested by increased mucosal permeability to hydrophilic macromolecules and increased bacterial translocation to mesenteric lymph nodes, also occurred at 20 h after CLP. Administration of human AM/AMBP-1 in established sepsis markedly attenuated tissue injury, reduced proinflammatory cytokine levels, ameliorated intestinal-barrier dysfunction, and improved the survival rate from 47% to 67%-80%. Thus, human AM/AMBP-1 can be further developed as a safe and effective therapy for patients with established sepsis.