A novel inhibitory peptide against blood coagulation factor IXa (FIXa) was isolated from the marine echiuroid worm (Urechis unicinctus, order Urechidae). U. unicinctus anticoagulant peptide (UAP), GELTPESGPDLFVHFLDGNPSYSLYADAVPR (M-w: 3344 Da) potently prolonged the activated partial thromboplastin time (APTT), corresponding to inhibition of an endogenous blood coagulation factor in the intrinsic pathway. In the specific factor inhibitory assay, FIXa activity in normal plasma was significantly (P < 0.05) decreased by addition of UAP in dose-dependant manner GC(50) = 42.6 mu g ml(-1)). Binding affinity assay using a surface plasmon resonance (SPR) spectrometer showed that UAP binding to FIXa could inhibit the interaction between FIXa and FX. The present results suggest that UAP bound to FIXa prolongs blood clotting time by inhibiting the conversion of FX to FXa in the intrinsic tenase complex. It is possible to provide biochemical properties and health benefits of a novel nutraceutical or pharmaceutical material with an anticoagulant activity. (c) 2007 Elsevier Ltd. All rights reserved.