It is now clear that vitamin K1 is part of a biochemical cycle that is essential for the conversion of specific bone peptides into a form that can bind calcium. We have used a recently described procedure for assaying vitamin K1 in plasma to test the involvement of this vitamin in fracture healing. Markedly depressed circulating levels were found in patients with fractures and the time taken for this level to return to normal appeared to be influenced by the severity of the fracture.