In a prospective, open, controlled clinical study, 190 consecutive patients who were thought to have bacterial peritonitis before operation, were randomised to antibiotic treatment during and after operation with either ceftriaxone 1 g plus metronidazole 1.5 g once daily (n = 94) or ampicillin 2 g plus netilmicin 150 mg twice daily plus metronidazole 1.5 g once daily (n = 96). Incisional and deep surgical wound infections, postoperative pneumonia and urinary tract infection as well as deaths caused by infection were recorded. Ceftriaxone-metronidazole was significantly more effective than ampicillin-netilmicin-metronidazole, 6/94 wound related infections (6%) compared to 18/96 (19%) (p = 0.02). In patients with peritonitis caused by a perforated colon or appendix the rates of clinical failure were 6% and 28%, respectively. We consider ceftriaxone plus metronidazole an efficient and easily administered antibiotic regimen in patients with bacterial peritonitis, and both the wide range of activity against Gram-negative aerobic rods and the long half life of ceftriaxone seem to be beneficial.