It is now established that vitamin K catalyzes the post-translational carboxylation of selected glutamyl residues in a prothrombin precursor to form y-carboxy glutamyl residues [l-3] . This modification enhances the Ca” and phospholipid binding capacity of the zymogen and permits its rapid conversion to thrombin in the presence of factors X, and V [4]. Since the vitamin K-deficient rat accumulates this precursor in its hepatic reticulum [5,6], unlike cow and man which secrete a des-y-carboxyprothrombin into the plasma [7,8], liver microsomes derived from vitamin K-deficient rats provide an ideal system for the study of vitamin K action in vitro. Shah and Suttie [9] first demonstrated the vitamin K-dependent conversion of precursor peptide to biologically active prothrombin, in liver microsomes from vitamin K-deficient rats, and this observatiorrhas been confirmed and extended in a number of laboratories [lo121. In this conversion, which requires NADH, oxygen, bicarbonate and vitamin K, H14C0i is fixed into peptide-bound r-carboxyglutamate. This membranous system has been solubilized by treatment with detergents [10,13,14] and a synthetic pentapeptide Phe-Ieu-Glu-Glu-Val, imitating residues 5-9 of bovine prothrombin precursor, has been reported to be an artificial substrate for the vitamin K-dependent carboxylation reaction [ 151. In this communication we wishto report the homologous pentapeptide, se-Leu-Glu-Glu-Be, based on the corresponding sequence of the prothrom-