We obtained a hybridoma using immune spleen cells from a mouse injected with human brain tissue factor that had been purified on a factor VII- agarose affinity column. This monoclonal IgG1, HTF1–7B8, inhibits tissue factor procoagulant activity. The concentration of HTF1–7B8 producing half-maximal inhibition is influenced by the concentration of factor VIIa, suggesting that the antibody and enzyme compete for the cofactor. The antibody was successfully used to detect both human and bovine tissue factor on nitrocellulose dot blots, indicating that the epitope recognized by this antibody is conserved in both species. This antibody clearly reveals tissue factor on a Western blot. An HTF1–7B8 affinity column was used to purify tissue factor from both human brain and placenta. The electrophoretic mobilities in polyacrylamide gels in the presence of sodium dodecyl sulfate (SDS) and the amino acid compositions of the purified tissue factor from brain and placenta are indistinguishable, as are their specific procoagulant activities in reconstituted systems. This antibody will be useful for immunopurification and characterization of tissue factor structure and mechanism.