Leukaemia inhibitory factor (LIF) and interleukin (IL)-6 are members of a cytokine group that share a common signal transducer gp130 and induce pleiotropic biological effects in cells of diverse lineage. In monocytes, LIF facilitates differentiation, which may stimulate the biosynthesis of tissue factor (TF) that initiates the coagulation cascade. We tested the hypothesis that LIF would enhance TF expression in human monocyte-derived macrophages (MDMs). Human peripheral blood mononuclear cells separated from whole blood by density centrifugation were allowed to differentiate into MDMs in primary culture, and were then exposed to LIF, IL-6 and oncostatin M (OSM) for 24 h. LIF and IL-6 receptors, and gp130 were demonstrated in MDMs by immunocytochemistry and RT-PCR. TF procoagulant activity (TF-PCA) was measured by recalcification clotting time and TF protein by Western blotting. The results show that both TF procoagulant activity and TF protein increased significantly in response to LIF over the concentration range of 1-100 nM (P < 0.03). Although OSM and IL-6 tended to enhance TF expression by MDMs, the increase did not reach statistical significance. Anti-LIF receptor and anti-gp130 antibodies attenuated the effect of LIF on TF expression as assayed by both bioassay and flow-cytometry. In conclusion, LIF increases TF-PCA and TF protein in MDMs, and specific anti-LIF receptor antibodies attenuate this effect. Thus, LIF may regulate by a gp130-dependent pathway macrophage-mediated procoagulant function in diverse pathological states involving inflammation and thrombosis and seems to serve as an important mediator at the interface between these processes.