We have examined the effects of seven proteases on human placental tissue factor in Triton X-100, focusing on extracellular and cytoplasmic domains recognized by monoclonal antibodies HTF1, C28 1.1, and C28 2.1. Plasmin produced peptides recognized on Western blots by C281.1 but not HTF1. None of the other proteases destroyed the extracellular epitope without also removing the cytoplasmic epitope, and both trypsin and chymotrypsin removed the cytoplasmic epitope with little effect on the extracellular domain. Proteinase K destroyed both epitopes, as did neutrophil elastase when used at a relatively high concentration. When digests were sampled over time and reconstituted with lipids for determination of tissue factor activity, only proteinase K consistently produced a loss in tissue factor activity at four hours. After 24 hr, other enzymes also decreased the recovered activity, with the order of effectiveness elastase > trypsin > chymotrypsin.