Protein C inhibitor (plasminogen activator inhibitor-3) is a plasma glycoprotein and a member of the serine proteinase inhibitor superfamily. In the present study, the human gene for protein C inhibitor was isolated and characterized from three independent phage that contained overlapping inserts coding for the entire gene. The genomic DNA was isolated and studied by restriction mapping, polymerase chain reaction analysis, and DNA sequencing. The gene was 11.5 kilobases in length and consisted of five exons separated by four introns. In addition, 0.8 kilobases of DNA from the 5'-flanking region were sequenced. The exon-intron boundaries all observed the "GT-AG" rule. The gene for protein C inhibitor was assigned to chromosome 14 by polymerase chain reaction analysis of human/hamster hybrid cell lines. The organization of the gene for protein C inhibitor is similar to the genes coding for alpha 1-antitrypsin and alpha 1-antichymotrypsin. The genes for these two proteins are also localized on chromosome 14 suggesting a recent evolution of the genes for these three proteins from a common ancestor.