The influence of a single intravenous dose of antibiotic on faecal flora and the emergence of Clostridium difficile was studied in volunteers. Seventy-eight volunteers (13 groups of 6 receiving 5 penicillins and 8 cephalosporins) were given a single intravenous dose of antibiotic. Results were compared with a control group of 6 volunteers who did not receive an antibiotic. Changes in the faecal flora were monitored over two weeks. Only cephalosporins were associated with emergence of Cl. difficile, penicillins and controls were not. Frequency of emergence of Cl. difficile in the 6 volunteers in each of the cephalosporin groups were as follows: cephaloridine 0, cephazolin, 1 cefuroxime 1, cefoxitin 2, cefotaxime 2, latamoxef 3, ceftriaxone 2, cefotetan 4. Latamoxef (moxalactam) was associated with a significant rise in total aerobic bacterial counts largely due to increased counts of enterococci. Cefotetan eliminated Escherichia coli and Bacteroides fragilis from the faecal flora three days after administration.