A survey of the medical literature reveals conflicting data as to whether bacteremia occurs during endoscopic examination of the lower gastrointestinal tract. In an attempt to study this problem a prospective study was undertaken, and the first arm of the study included patients undergoing colonoscopy. The second arm of the study, comprising patients undergoing proctosigmoidoscopy, will be presented subsequently. Fifty-one patients undergoing colonoscopy were studied. Excluded from the study were patients with elevation of temperature above 101 degrees F, inflammatory bowel disease, diarrhea, valvular heart disease, vascular prosthesis, chemotherapy, or immunosuppression. Aerobic and anaerobic culture were done before and after the procedure, as well as at timed intervals during the procedure. Whenever biopsy or polypectomy were carried out, further cultures were done. Skin cultures were done from venipuncture sites. In one patient (2 per cent) Staphylococcus epidermidis was found in more than one set of cultures. Polypectomy or biopsy were not associated with bacteremia.