We developed an ELISA to quantitate complexes of activated protein C (APC) with a major plasma APC inhibitor, alpha 1-antitrypsin (alpha 1AT) in human plasma based on the sandwich principle using two different antibodies directed towards protein C and alpha 1AT, respectively. This ELISA test was specific for APC:alpha 1AT complexes and sensitive to greater than or equal to 150 pg complex. Fifty-one of 56 healthy donors had APC:alpha 1AT complex levels above the detection limit (3 ng/ml) ranging from 4 to 14 ng/ml (mean value +/- SD: 7.6 +/- 2.5 ng/ml). Patients (n = 10) with disseminated intravascular coagulation (DIC) had detectable levels of APC:alpha 1AT complex ranging from 21 to 125 ng/ml (median: 69 ng/ml). Complexes of APC with plasma protein C inhibitor (PCI) were also measured using an ELISA sandwich assay. None of the 30 healthy donors had detectable levels (greater than or equal to 5 ng/ml) of APC:PCI complex, and plasma samples from 9 of 10 DIC patients had detectable concentrations of APC:PCI complex ranging from 10 to 63 ng/ml (median: 22 ng/ml). APC:alpha 1AT complex was detected in 25 of 26 patients with deep venous thrombosis (DVT), with levels ranging from 5 to 136 ng/ml (median: 23 ng/ml), whereas APC:PCI was detected in only 6 DVT patients, with levels between 11 and 105 ng/ml. PCI antigen levels in 70 normals ranged from 56 to 175% (mean +/- SD: 99.1% +/- 24.2%). PCI antigen levels were decreased in DIC patients, in patients with cerebral arterial thrombosis, and in DVT patients undergoing heparin therapy, but not in patients with myocardial infarction. PCI antigen levels were decreased much further in DVT patients receiving heparin compared to those not receiving heparin, showing that heparin therapy is associated with a decrease in PCI levels. The detection in normal subjects and in thrombotic patients of circulating APC:inhibitor complexes supports the view that the protein C pathway is activated during DIC and DVT. Moreover, it emphasizes that both PCI and alpha 1AT are physiologic inhibitors of APC. Thus, measurement of APC complexes may provide sensitive parameters for specific detection of activation of the clotting and protein C pathways.