Expression of tissue factor, the initiator of the extrinsic coagulation protease cascade, is a feature of certain malignant tumours. To study the modulation of tissue factor expression we incubated the breast cancer cell line MCF-7 with several growth factors. Epidermal growth factor (EGF), transforming growth factor alpha (TGF alpha) and interleukin-1 (IL-1) rapidly increased tissue factor expression of MCF-7 cells peaking at 6-8 h after starting point of incubation, as determined by clotting test, enzyme linked immunosorbent assay and flow cytometry. The data presented support the hypothesis that modulation of constitutive tissue factor expression in tumour cells by TGF alpha and IL-1 could also occur in vivo possibly resulting from interactions of stromal and cancer cells. The meaning for tumour biology, however, remains unclear.