The use of the previously described electrofluorimetric reaction detection method for phylloquinone (vitamin K1) was extended to the determination of vitamins K1 epoxide in human plasma. The limit of detection for vitamin K1 epoxide is 60 pg, corresponding to plasma concentrations of about 120 pg/ml. The isolation method involves a liquid-liquid extraction of the lipids from the plasma, followed by a clean-up with silica. Synthesized 2',3'- dihydrovitamin K1 is used as an internal standard. The method was applied to the study of the presence of vitamin K1 epoxide and phylloquinone in plasma samples from healthy volunteers and from patients undergoing anticoagulant therapy.