Abstract: Polypeptides are disclosed that are capable of being isolated from ectoparasitic leeches which inhibit the alternative route of complement activation but which have substantially no effect on complement activation by the classical route.

Abstract: The inhibitors, obtainable from tissue or secretions of leeches typically of the order Rhynchobdellida, has the following terminal sequence: NH.sub.2 -Lys-Leu-Leu-Pro-Cys-Lys-Glu-Y-His-Gln-Gly-Ile-Pro-Asn-Pro-Arg- wherein Y represents any amino acid sequence; or a pharmaceutically acceptable salt, derivative or bioprecursor of said sequence, or an analogue or homologue thereof. Because of their extreme potency in the nanomolar range, they can be used to treat a number of diseases where protein cross-linking is important. They can be used for the treatment of Crohn's disease, tumor implantation, atherosclerosis, thrombotic microangiophathy, fibrous growths of the skin, acne, scar formation, membranous glomerulonephrits, cataracts, or infection with microfilarial nematodes. In particular, they can be used to reduce the stability of thrombi so that they are more susceptible to lysis by thrombolytic agents.