Protein C is a precursor to a serine protease that is present in mammalian plasma. In its activated form, it readily inactivates factor Va and factor VIIIa, two proteins that participate as cofactors in the blood coagulation cascade. In the present studies, a lambda gt11 library containing cDNA inserts prepared from human liver mRNA has been screened with an antibody to human protein C. Seven positive clones were isolated from 2 X 10(6) phage and were plaque-purified. The cDNA inserts of two of these phage were sequenced and shown to code for human protein C. Each cDNA insert coded for a portion of the light chain of the molecule, a connecting region, the heavy chain, a stop codon, a 3'-noncoding region, and a poly(A) tail. The length of the noncoding sequence on the 3' end differed in the two clones, but each contained a processing or polyadenylylation signal followed by a poly(A) tail. The amino acid sequence as determined from the cDNA indicates that protein C is synthesized as a single-chain polypeptide containing the light chain and the heavy chain connected by a dipeptide of Lys-Arg. The single-chain molecule is then converted to the light and heavy chains by cleavage of two or more internal peptide bonds. In plasma, the heavy and light chains of protein C are linked together by a disulfide bond. The amino acid sequence of human protein C shows a high degree of homology with that of the bovine molecule. The DNA sequence coding for the catalytic region near the active site serine in human protein C also showed a high degree of DNA and amino acid sequence identity with prothrombin, factor IX, and factor X, three of the other vitamin K-dependent serine proteases that are present in plasma.