In this study we have looked for differences in coagulation parameters before and after surgical removal of benign or malignant tumours. A striking increase in thromboplastin activity of the blood monocytes was seen 1 d after surgery. This paralleled a fall in factor VII activity. At the same time, the sensitivity of the blood monocytes to stimulation by endotoxin increased significantly. We propose from this study that monocyte thromboplastin may be a postoperative thrombogenic factor. Increased levels of factor VIII and fibrinogen 2–3 d postoperatively were found, probably caused by inflammation reactions induced by the surgery. No difference in coagulation parameters could be demonstrated between patients with benign, noninvasive lesions and patients with invasive, carcinomatous lesions.