Protein C inhibitor was purified from human plasma by use of a dermatan sulfate or heparin column, followed by hydroxyapatite, gel filtration and ion exchange columns. A dimer of protein C inhibitor was detected by SDS-PAGE under reducing conditions, in addition to two forms of monomer species. One of the monomers, 52-kDa PCI, formed a stable complex with activated protein C, urokinase, plasma and tissue kallikrein, but the dimer species and 48-kDa PCI were inactive. When the monomer and dimer forms of protein C inhibitor were applied to 2D-PAGE, more than 20 spots were observed by Western blot analysis and were confirmed to be protein C inhibitor by MALDI-TOF mass spectrometry. The heterogeneity of the protein C inhibitor species was not due to glycosylation or phosphorylation.