Homozygous or double heterozygous protein-C deficiency can present at birth with purpura fulminans or later in life with venous thrombosis. Two homozygous patients who had previously sustained thrombotic episodes were investigated at a time when they were asymptomatic and not receiving antithrombotic therapy. The plasma levels of protein-C antigen and activity in both individuals were approximately 20% of normal. We administered a highly purified plasma-derived protein C concentrate to these individuals and monitored levels of several markers of in vivo coagulation activation. Assays for protein-C activation (activated protein C and protein C activation peptide) showed a sustained increase from reduced baseline levels, whereas thrombin generation (as measured by prothrombin fragment F1 + 2) gradually decreased over about 24 hours into the normal range. These investigations provide direct evidence that protein C is converted to activated protein C in vivo, and that the protein-C anticoagulant pathway is a tonically active mechanism in the regulation of hemostatic system activation in humans.