Hirullin P18 is a 61-amino acid hirudin-related protein having potent antithrombin activity. Similar to hirudin, it contains a highly acidic C-terminus, but has a significantly different sequence from any other known hirudin variant. The present study demonstrates that the C-terminal fragment acetyl-hirullin Pl8,,,,, p assesses an antithrombin potency similar to that of acetyl-desulfatohirududirL,,,. Additionally, like the hirudin fragment analog, it inhibits fibrin-clot formation by binding to a non-catalytic site on thrombin. Sequential shortening of the hirullin P18 C-terminal fragment demonstrates the critical nature of PheSi, which corresponds to the important Phd6 residue of hirudin. Although the sequences of hirullin Pl8,,, and hirudin,,, have substantial differences, the C-terminal functional domain represented by hirullin Pl8,,,, appears to be comparable to hirudins,,, in terms of its functional role in antithrombin activity.Hirullin P 18; Hirudin; Synthetic peptide; Thrombin