1. Through the vitamin K 1 cycle, phylloquinone is now known to play an active role, not only in relation to prothrombin, but also in the synthesis of bone peptides. 2. The recent development of a sensitive method allowed the demonstration of a deficit of vitamin K 1 in the circulation of osteoporotic subjects. 3. Vitamin K 2 , namely the menaquinones of various chain-lengths, has been shown by others to be more effective than vitamin K 1 in the curative rat bioassay. 4. Earlier reports had shown that the concentration of menaquinones in human liver may exceed that of vitamin K 1 . But previous methods were too insensitive for testing the normal circulating levels of menaquinones in the human. 5. The new sensitive method has now been applied to measuring the circulating levels of vitamin K 1 and of two of the menaquinones, namely menaquinone-7 and menaquinone-8. 6. In normal individuals, the circulating levels of vitamin K 1 were the same, irrespective of age. 7. In young normal subjects, the combined levels of menaquinone-7 and menaquinone-8 were at least the same as the level of vitamin K 1 . In elderly normal subjects, there was a marked deficit of menaquinone-8.