A cDNA library in lambda-phage lambda gt11 containing DNA inserts prepared from human liver mRNA was screened with monoclonal antibodies to human protein C inhibitor. Six positive clones were isolated from 6 X 10(6) phages and plaque purified. The cDNA in the phage containing the largest insert, which hybridized to a DNA probe prepared on the basis of the amino-terminal amino acid sequence of the mature inhibitor, was sequenced. This cDNA insert contained 2106 base pairs coding for a 5'-noncoding region, a 19-amino acid signal peptide, a 387-amino acid mature protein, a stop codon, and a long 3'-noncoding region of 839 base pairs. Based on the amino acid sequence of the carboxyl-terminal peptide released by cleavage of protein C inhibitor by activated protein C as well as by thrombin, the reactive site peptide bond of protein C inhibitor is Arg354-Ser355. Five potential carbohydrate-binding sites were found in the mature protein. The high homology of the amino acid sequence of protein C inhibitor to the other known inhibitors clearly demonstrates that protein C inhibitor is a member of the superfamily of serine protease inhibitors including alpha 1-antichymotrypsin, alpha 1-antitrypsin, antithrombin III, ovalbumin, and angiotensinogen. Based on the difference matrices for these proteins, we present possible phylogenetic trees for these proteins.