Protein C inhibitor is a plasma protein whose ability to inhibit activated protein C, thrombin, and other enzymes is stimulated by heparin. These studies were undertaken to further understand how heparin binds to protein C inhibitor and how it accelerates proteinase inhibition. The region of protein C inhibitor from residues 264-283 was identified as the heparin-binding site. This differs from the putative heparin-binding site in the related proteins antithrombin and heparin cofactor. The glycosaminoglycan specificity of protein C inhibitor was relatively broad, including heparin and heparan sulfate, but not dermatan sulfate. Non-sulfated and non-carboxylated polyanions also enhanced proteinase inhibition by protein C inhibitor. Heparin accelerated inhibition of alpha-thrombin, gamma T-thrombin, activated protein C, factor Xa, urokinase, and chymotrypsin, but not plasma kallikrein. The ability of glycosaminoglycans to accelerate proteinase inhibition appeared to depend on the formation of a ternary complex of inhibitor, proteinase, and glycosaminoglycan. The optimum heparin concentration for maximal rate stimulation varied from 10 to 100 micrograms/ml and was related to the apparent affinity of the proteinase for heparin. There was no obvious relationship between heparin affinity and maximum inhibition rate or degree of rate enhancement. The affinity of the resultant protein C inhibitor-proteinase complex was also not related to inhibition rate enhancement, and the results showed that decreased heparin affinity of the complex is not an important part of the catalytic mechanism of heparin. The importance of protein C inhibitor as a regulator of the protein C system may depend on the relatively large increase in heparin-enhanced inhibition rate for activated protein C compared to other proteinases.