A highly sensitive method for measuring endogenous vitamin K1, menaquinone-4 (which is one of the Kz vitamins) and vitamin K1 2,3-epoxide in human pIasma was developed, based on high-performance liquid chromatography with coulometric reduction and fluorimetric detection, following extraction from plasma and purification on a Sep-Pak silica cartridge. The detection limits of vitamin K1, menaquinone-4 and vitamin K1 2,3-epoxide were 5, 5 and 8 pg per injection for the standard substances and 30.30 and 50 pg/mI in human plasma, respectively.