The amino acid sequence of the heavy chain of protein C from bovine plasma is reported together with supporting data. The heavy chain contains 260 amino acid residues and it has three carbohydrate side chains. Its structure was determined by sequenator degradation of fragments obtained by enzymatic and chemical cleavage. The sequence is homologous with those of the other serine proteases. On activation, an NH2-terminal tetradecapeptide is removed.