The faecal excretion of short-chain fatty acids (SCFAs) has been measured in groups of six healthy subjects before, during, and after they received the antibiotics clindamycin, ampicillin, or metronidazole perorally for 6 days. Intake of clindamycin reduced the median total concentration of SCFAs from 62.9 mmol/kg faeces (wet weight) to 7.3 mmol/kg (p less than 0.05). During therapy the relative amounts of acetic acid increased from 50% to 90% of the total concentration (p less than 0.05). Ampicillin reduced the median SCFAs concentration from 62.4 mmol/kg to 47.8 mmol/kg (p less than 0.05), whereas metronidazole did not change the SCFAs concentrations significantly. The SCFAs concentrations returned to normal within 5 weeks after the treatment in all subjects. Clindamycin was detected in high concentrations in faeces during therapy. Ampicillin was detected in only one faecal sample, which was from the only subject in the ampicillin group without detectable beta-lactamase activity in faeces. Metronidazole could not be detected in faeces from any subjects receiving this drug. Clindamycin and ampicillin, but not metronidazole, induce pronounced changes in faecal SCFAs, most likely reflecting severe changes in the colonic ecosystem. An antibiotic's influence on the colonic microflora may in part depend on its antimicrobial spectrum and the concentration of antimicrobially active drug in the gut.