A new potent inhibitor of complement system, named complestatin, was isolated from the mycelium of Streptomyces lavendulae SANK 60477. Complestatin (C₆₁H₄₅N₇O₁₅Cl₆, MW 1, 325) was a peptide compound having two unusual amino acids, D-(-)-4-hydroxyphenylglycine and D-(-)-3, 5-dichloro-4-hydroxyphenylglycine. This compound inhibited the hemolysis of sensitized sheep erythrocytes (EA) mediated by guinea pig and human complement 50% at concentrations of 0.4 and 0.7 μg/ml, respectively, but did not trypsin and α-chymotrypsin activities at 200 μg/ml. When complestatin was administered intravenously to the sensitized guinea pigs, it strongly inhibited the systemic anaphylactic shock elicited by the antigen probably by blocking generation of anaphylatoxins (C3a and C5a).