Culture for bacteria and assays for endotoxin were performed on specimens of mesenteric and peripheral venous blood from eight patients with ulcerative colitis who underwent surgery for severe uncontrolled disease. No significant bacteraemia occurred in either portal or systemic blood. Systemic endotoxaemia developed in three patients during surgery but occurred before bowel mobilisation only in the one patient whose colitis was complicated by hyposplenism. Mesenteric endotoxaemia occurred in only three patients before bowel mobilisation, but was detected during surgery in two of the three patients who developed systemic endotoxaemia. We conclude that, contrary to earlier reports, portal bacteraemia must be infrequent in ulcerative colitis. Systemic endotoxaemia does, however, occur in a significant proportion of cases during colectomy. Although in the patients studied this led to no clinical problems, it is likely to have been the precipitating factor for the syndrome of disseminated intravascular coagulation (DIC) that we have previously observed after colectomy in some of our patients with hyposplenism secondary to inflammatory bowel disease.