Enzyme-linked immunosorbent assays (ELISA's) were developed for the measurement of protein C inhibitor (PCI) antigen and activity and for its complexes with activated protein C (APC) in plasma. For PCI activity and antigen, APC or anti-PCI, respectively, was immobilized to microtiter plates and PCI bound was detected with labelled anti-PCI antibodies. For APC:PCI complexes, two different antibodies directed against protein C and PCI were used. The assays for PCI were calibrated with pooled normal human plasma (NHP) and with purified PCI, and for APC:PCI complexes with known concentrations of purified pre-formed complexes added to buffer or to plasma. The lower limit of sensitivity of the PCI activity and antigen assays was 10 ng/ml and 0.5 ng/ml, respectively and for plasma APC:PCI complexes 12 ng/ml. Mean coefficients of variation of 1.5% to 5.8% (intra-assay) and 2.1% to 9.8% (inter-assay) were found for the assays. For PCI antigen, a range of 56% to 162% of the NHP value was obtained in samples from 70 healthy donors (mean +/- SD = 98.6% +/- 23.1%). For PCI activity, the range was 59% to 148% (94.3% +/- 20.2). A good correlation (0.92) was obtained when both assays were compared. Plasma levels of APC:PCI complexes in 30 normals were under the detection limit (less than 12 ng/ml). In plasma samples from 10 patients with disseminated intravascular coagulation (DIC) PCI antigen concentrations were decreased (55.6% +/- 20%) and 8 of the patients had APC-PCI complex levels between 32 and 240 ng/ml (median, 35 ng/ml). After addition of 20 micrograms/ml APC to NHP or to protein C depleted plasma, 6.1 micrograms/ml complexes were recovered after 90 min incubation. Incubation of 10 micrograms/ml APC with NHP in the presence of 10 U/ml heparin yielded 11 micrograms/ml complexes after 90 min, which represent more than 90% of the maximum possible value. Thus, the method should be adequate to study complexes of APC in vivo in clinical conditions in which activation of protein C pathway may occur.