A pilot study of 20 patients who underwent colonoscopy was performed to investigate the occurrence of endotoxemia related to the procedure and its clinical significance. With the use of the limulus lysate method of assay, endotoxemia was demonstrated in 25 per cent of the patients during colonoscopy and 65 per cent afterwards, but was not associated with either concomitant bacteremia or any ill effects. The appearance of endotoxin in significant concentration in the presence of normal liver function and with no overt focus of infection, suggests that substantial absorption of bacterial products which originate in the intestine occur, but under normal circumstances cause no demonstrable deleterious effects.