Abstract: The present invention relates to biologically active peptides which specifically inhibit the Factor IX-dependent pathway of blood coagulation. These peptides are based on a specific motif which represents a sequence of Factor VIII (amino acid 1811-1818) involved in binding to activated Factor IX as well as to the intact Factor IX zymogen. Binding of said peptides to Factor IX or activated derivatives thereof effectively inhibits complex assembly with Factor VIII. These peptides thereby specifically interfere in the Factor IX-dependent pathway of thrombin formation, while leaving other haemostatic pathways unaffected. By virtue of this unique specificity, the peptides of this invention are particularly useful in novel pharmaceutical compositions for the treatment of thrombotic disorders.