Compstatin is a 13-residue cyclic peptide and a potent inhibitor of the complement system, and has species specificity. In Vitro: Compstatin binds to baboon C3 and is resistant to proteolytic cleavage in baboon blood (similar to humans); Compstatin inhibits only the activation of primates' complement system^[1]. In Vivo: Compstatin (21 mg/kg) produces complete inhibition when given as a combination of bolus injection and infusion. Compstatin completely inhibits in vivo heparin/protamine-induced complement activation without adverse effects on heart rate or systemic arterial, central venous, and pulmonary arterial pressures^[1]. Pig xenografts survival is significantlylonger in the Compstatin perfused group than in the control group. At rejection the biopsy samples obtained from kidneysperfused with Compstatin show less deposition of C3 and fibrin than do the control kidneys^[2].