Background - Activation of blood coagulation is a common complication of cancer and inflammation in both humans and experimental animals. Increased production of tissue factor-the principal initiator of the coagulation process - by endothelial cells, monocytes, and macrophages has been implicated in these conditions. Aim - To investigate whether urinary tissue factor (uTF) might reflect the state of monocyte/macrophage activation and be a useful diagnostic test. Methods - Urine was centrifuged at 51 000 g to sediment tissue factor containing membrane vesicles. The tissue factor was then solubilised in beta-octyl-glucopyranoside and assayed in a specific chromogenic assay adapted for use in microtitre plates. Results-The assay proved tot be sensitive, specific, and reproducible. The normal range of uTF was relatively narrow and unaffected by age, sex, or cigarette smoking. Levels were not significantly influenced by storage of urine samples before assay or by the presence of fresh blood in the urine sample. Conclusions-This method may have diagnostic application in the study of haemostatic activation in patients with cancer and other disease states.