The isolation and synthesis of vitamin K1(2-methyl-3-phytyl-1,4-naphthoquinone) in August of 1939 by Fieser and his associates,1by Doisy and his co-workers2and by Almquist and Klose3gave new impetus to the clinical evaluation of the antihemorrhagic activity of synthetic vitamin K1and its chemical derivatives in hypoprothrombinemia. It has been amply established by Ansbacher and his associates4and by others5that 2-methyl-1,4-naphthoquinone and more recently that 2-methyl-1,4-naphthohydroquinone are more rapidly acting and have greater potency than the many other derivatives of vitamin K1thus far elaborated in the biochemist's laboratory. It is our purpose in this paper to report our experience with the intravenous use of 2-methyl-1, 4-naphthoquinone for patients suffering from hypoprothrombinemia. CHEMISTRY Numerous chemical compounds closely related to the naturally occurring vitamin K1of alfalfa and K2of putrefied fish meal have been made available for clinical research,