The induction of procoagulant activity (PCA) by human recombinant tumor necrosis factor (rTNF) was studied in human monoblastic leukemia cell line U937 and human peripheral blood monocytes. Using a one-step recalcificating clotting assay, PCA in cell lysates or whole cell preparations was measured by comparison to a rabbit brain thromboplastin standard. There was a dose- and time-dependent increase in PCA when U937 cells were cultured with rTNF. The effect of rTNF was not enhanced by recombinant human interferon-gamma (rIFN gamma). Cycloheximide inhibited the expression of PCA by U937 cells, showing that protein synthesis was necessary to mediate the effects of rTNF. Whole cell preparations demonstrated that greater than 80% of the PCA was expressed on the surface of the cells. The PCA functioned as a tissue factor-like substance, since it required coagulation factor VII and factor X. rTNF also increased PCA in human monocytes in a dose- and time-dependent manner. This effect was abrogated by boiling the rTNF for ten minutes, and was not inhibited by adding polymyxin-B to the cultures, making it unlikely that endotoxin accounted for the observed effects. These results suggest that TNF-induced expression of tissue factor by mononuclear phagocytes may modulate immunologic, inflammatory, and hemostatic processes.