Publisher Summary Vitamin K activity is shared by a fairly large number of compounds. They consist, in the main, of two naturally occurring groups of compounds: phylloquinones (K), containing saturated isoprenoid side chains, and menaquinones (MK) with unsaturated isoprenoid side chains. High biological activity, however, is also found in several simple 2-methylnaphthaquinones of which menadione is the best known substance. The activity of these various forms of vitamin K not only is affected by their chemical structure that may affect their absorption and metabolism but may be inhibited to varying degrees by a number of other substances and environmental factors. Vitamin K deficiency in animals is characterized by a defect in the blood coagulation process, as a result of which subcutaneous and intramuscular hemorrhages occur throughout the body, eventually causing death. The deficiency has been produced in several mammalian and avian species but is probably not observed in the natural state. Furthermore, under many conditions, the intestinal microflora of the rat appears to synthesize enough vitamin K for the animal's needs. Deficiency disease is more easily produced in avian species, such as the chick, that is the preferred experimental animal.