The continuous chromogenic tissue factor assay has been compared to one-stage and two-stage coagulation assays, and its sensitivity has been established using purified human placental tissue factor. Log-log plots of the clot-times versus A405/min2 are linear over the mutually useful ranges of tissue factor concentration. Using human placental tissue factor reconstituted into phosphatidylserine-phosphatidylcholine (30:70) vesicles, the chromogenic assay quantitatively detected tissue factor in dilutions containing from 3 femtomoles to less than 10 attomoles of protein.