Progressive inhibition of tissue factor activity occurs upon its addition to human plasma (serum). This process requires the presence of factor VII(a), facto-X(a), Ca2+, and another component in plasma that we have called the tissue factor inhibitor (TFI). A TFI secreted by HepG2 cells (human hepatoma cell line) was isolated from serum-free conditioned medium in a four-step procedure including CdCl2 precipitation, diisopropylphosphoryl-factor Xa affinity chromatography, Sephadex G-75 superfine gel filtration, and Mono Q ion-exchange chromatography. The purified TFI contained a predominant band at Mr 38,000 on NaDodSO4/polyacrylamide gel electrophoresis that comigrates with inhibitory activity. Like the activity present in plasma, this TFI requires the presence of factor VII(a), factor X(a), and Ca2+ to express inhibitory activity. Its specific activity (assuming an extinction coefficient of 10 at 280 nM, for a 1-cm path length through a 1% solution) was 9800 units/mg of protein, where 1 unit of TFI activity was defined as that present in 1 ml of normal pooled serum.