Peptides (residues 1-42) (bovine prothrombin numbering) from bovine Factor X1 and X2 have been separately purified and digested before mass-spectrometric sequence assignment and identification of gamma-carboxyglutamic acid. N-terminal sequence was found to be identical, and 12 residues of gamma-carboxyglutamic acid were unambiguously identified. The new data give conclusive evidence for the N-terminal primary structure of bovine Factor X and extend present knowledge to show (i) unambiguous assignment of gamma-carboxyglutamic acid residues, including a previously unreported residue of gamma-carboxyglutamic acid at position 40 in both Factor X1 and X2, (ii) the physical difference between Factors X1 and X2 is not due to either different amino acid sequences or different gamma-carboxyglutamic acid contents of the N-terminal 42 residues.