Aztreonam, ceftazidime, cefoperazone and latamoxef (moxalactam) were evaluated in a hamster model for antibiotic-associated colitis. Aztreonam, a novel monocyclic beta-lactam antibiotic specifically directed against aerobic Gram-negative bacteria with limited activity against Gram-positive and anaerobic bacteria did not cause haemorrhagic caecitis and death in hamsters when administered intraperitioneally or orally. Quantitative caecal cultures showed no changes in the anaerobic caecal microflora of hamsters treated intraperitoneally with aztreonam and only a temporary decrease in anaerobic Gram-negative bacilli in hamsters treated orally. Clostridium difficile and its cytotoxin were not present in these animals. Parenteral administration of ceftazidime also did not affect the anaerobic caecal microflora or cause caecitis. However, when given orally, ceftazidime suppressed the anaerobic caecal microflora and a lethal C. difficile-induced caecitis developed. Latamoxef or cefoperazone given parenterally resulted in caecitis and death in five days with marked changes in the anaerobic caecal microflora and C. difficile present in the caecal contents of these animals. The lethal haemorrhagic caecitis observed in this study was indistinguishable from that seen in the clindamycin-induced colitis model in the hamster. The possible correlation of these findings to gastrointestinal disturbances as a complication of antibiotic therapy in humans is discussed.