FUT-175 (nafamstat mesilate) is a novel synthetic protease-inhibiting agent which has potent, reversible and selective inhibitory activity against trypsin-like serine proteases. A determination method for FUT-175 in biological materials, particularly in blood, was established based on the potent trypsin-inhibitory activity of this drug. The procedure consists of two steps : treatment of blood specimens with formic acid-containing ethanol for deproteinization and extraction of FUT-175 with minimal hydrolysis, and a clean-up with n-pentane for removal of remaining blood constituents such as phospholipids, fats and any fluorescent materials. This method shows good reproducibility and sensitivity ; the detection limit was as low as 1 ng of FUT-175/ml blood.