Patients treated with antibiotics develop diarrhea that, in most cases, is self-limiting. However, in some instances, particularly in those patients treated with ampicillin or clindamycin, a severe, life-threatening pseudomembranous enterocolitis develops. This disease has characteristic histopathology, and membranous plaques can be seen in the colon by endoscopy. Pseudomembranous enterocolitis and antibiotic-associated diarrhea are caused by an anaerobic gram-positive rod, C. difficile. It has been recently shown that C. difficile produces a protein toxin with a molecular weight of about 250,000. The "toxin" is, in fact, two toxins, toxin A and toxin B. Both toxins are always present in fecal samples, but there is approximately 1000 times more toxin B than toxin A. Toxin A has enterotoxic activity-that is, it elicits a positive fluid response in ligated rabbit ileal loops-whereas toxin B appears to be primarily a cytotoxin. The Bacteroides and clostridium organisms are anaerobic and can be found in the intestinal tract. Also, these would be killed by the antibiotics given to the patient. Staphylococcus aureus is resistant to many antimicrobials and can cause gastroenteritis if it becomes predominant, but it usually does not cause as serious disease as pseudomembranous enterocolitis. Commercial laboratory tests are available to identify C. difficile toxin and enterotoxin.