Protein C inhibitor (PCI) is a heparin-dependent serpin present in a native form in plasma at concentrations of 5 micrograms/mL. In vitro, PCI inhibits activated protein C (APC), thrombin, plasma kallikrein (KK) and urokinase-(uPA) and tissue-type plasminogen activator (tPA), and we have shown in vivo inhibition of APC, uPA and KK by PCI. In order to further characterize the physiological role of PCI, we have measured the level of PCI in several biological fluids. PCI antigen was assayed by ELISA and PCI activity was measured by its capability to form complexes with APC in the presence of heparin. Seminal plasma from voluntary donors had PCI levels (160 +/- 20 micrograms/mL, mean +/- SD) about 30 or 40 times higher than those found in blood plasma. Patients under a fertilization program had significantly reduced PCI seminal levels (110 +/- 35 micrograms/mL). Seminal plasma PCI retained about 45% of its activity immediately after ejaculation, and the activity rapidly decreased following incubation of seminal plasma at 37 degrees C, in parallel with the appearance of complexes of PCI with prostate-specific antigen (PSA). PCI was present in seminal vesicle secretion, obtained by autopsy, at concentration similar to that observed in semen, was mostly active and was not inactivated by incubation of secretion at 37 degrees C. The mean functional and antigen levels of PCI in urine from normal donors were 0.58 and 0.25 micrograms/mL, respectively, whereas in saliva these levels were 20 and 0.8 ng/mL, respectively. Amniotic fluid contained PCI antigen levels of 2.1 +/- 0.2 microgram/mL. These results show that PCI is secreted in the seminal vesicles in a functional form, and suggest that PSA, a major secretory component of the prostate, is responsible for its inactivation. They also suggest a physiological role of PCI in reproduction, and show that PCI is present in various biological fluids.