Given that factor VIIa-tissue factor (TF) probably initiates coagulation in vivo, this study investigated the relationship between plasma concentrations of factor VIIa and prothrombin fragment 1 + 2 in plasma (the latter as an index of prothrombinase activity in vivo). The relationships between these two parameters and the concentrations of tissue factor pathway inhibitor (TFPI) and factor Xa-TFPI in plasma were also investigated. TFPI inactivates factor Xa in a reaction accelerated by heparin, whereas factor Xa-TFPI inactivates factor VIIa-TF and prothrombinase. Established enzyme-linked immunosorbent assays (ELISAs) were used to quantify TFPI and prothrombin fragment 1 + 2, whereas we developed an ELISA to quantify factor Xa-TFPI using affinity purified rabbit (anti-human TFPI)-IgG and chicken anti-(human factor Xa-TFPI)-IgY as the capture and detector antibodies, respectively. Plasma factor VIIa was quantified using truncated tissue factor. The concentrations of factor VIIa and prothrombin fragment 1 + 2 increased in parallel in the plasmas of up to 145 healthy adults assayed (P = 0.007), as did the concentrations of factor VIIa and TFPI (P = 0.0039), and prothrombin fragment 1 + 2 and TFPI (P = 0.013). In contrast, there was an inverse relationship between the concentrations of free factor Xa-TFPI and factor VIIa (P less than 0.0001) and free factor Xa-TFPI and prothrombin fragment 1 + 2 (P=0.0095). These results are consistent with factor Xa-TFPI regulating factor VIIa-tissue factor and prothrombinase in vivo.