Proteolytic and leucine aminopeptidase activity levels in faeces from patients with Crohn's disease and healthy subjects were compared and were found to be 2.5 and 6 times higher, respectively, in patients with Crohn's disease. Patients who had undergone partial bowel resection even showed a 12- and 40-fold increase of enzyme activities. A part of the elevated levels of faecal proteases probably results from ileal malabsorption. This was supported by studies in germ-free and conventional rats showing that the ileum is the major site of absorption of digestive proteolytic enzymes and leucine aminopeptidase. The other part of the increased protease activity was attributed to the abnormal anaerobic faecal flora of patients with Crohn's disease. Interactions between flora and enzymes were studied in rats. The resident flora of the caecum was found to stimulate absorption, or to inactivate the major part of the remaining enzymes. The elevated levels of faecal proteases in Crohn's disease may play a role in the damage of the intestinal mucosa.