The amino acid sequence of protein Z has been determined from sequence analysis performed on fragments obtained by chemical and enzymatic degradations. The polypeptide consists of a single chain containing 396 amino acid residues (Mr 43 677). Comparison with the vitamin K‐dependent plasma proteins reveals an extensive homology. The N‐terminal part, containing 13 γ‐carboxyglutamic acid and one β‐hydroxyaspartic acid residue, is extensively homologous to and of similar length to the light chain of factor X. The remainder of protein Z is homologous to the serine proteases and of similar size to the heavy chain of factor Xa, but of the active site residues only aspartic acid‐102 is present. Histidine‐57 and serine‐195 are replaced in protein Z by threonine and alanine, respectively. The physiological function of protein Z is still uncertain.