Peritoneal lavage with ten hourly repeated instillation-drainages of one litre lavage fluid containing 50 mg doxycycline (Vibramycin Pfizer) was carried out on six patients suffering from diffuse peritonitis due to perforated appendicitis. Bacteriological specimens were obtained both from the perforation in the appendix and the peritoneal fluid at operation and the doxycycline concentrations in blood and lavage fluid were determined at intervals up to 72 hours postoperatively. All the patients recovered, but the lavage was considered a failure in one case as a small abscess and multiple adhesions were found at reoperation 1 1/2 months later. Bacteriological swabs from the perforated appendix and the peritoneal fluid usually contained the same bacteriological species, and as a rule several species were isolated. Doxycycline compared favourably with ampicillin as regards bacterial sensitivity, but the difference was too slight to be of practical importance. Doxycycline was absorbed very rapidly from the peritoneal cavitiy, and due to its slow excretion a cumulation occurred. In this small series doxycycline did not appear to give any real advantage over ampicillin, but it may be considered the drug of choice in patients with penicillin allergy, and in those who fail to respond to ampicillin.