The separation of sufficient cis and trans forms of vitamin K for feeding and metabolic studies was accomplished on silica gel columns eluted with solvent containing n-butyl ether. The lack of biological activity of the cis isomer of phylloquinone was observed. The cis isomer was retained longer in liver, particularly in mitochondria, but had low retention in that portion of the endoplasmic reticulum isolated as the rough membrane fraction. The cis isomer of phylloquinone was a poor substrate for 2,3-epoxidation in vivo and in vitro. These data are consistent with the hypothesis that epoxidation of vitamin K is coupled to the biological activity of the vitamin, and that microsomes are the site of metabolism and function of vitamin K.