Faecal excretion of short-chain fatty acids (SCFAs) has been measured by gas chromatography in groups of six or seven healthy subjects before, during, and after they received the antibiotics bacitracin, co-trimoxazol, doxycycline, erythromycin, nalidixic acid, ofloxazin, or vancomycin orally for 6 days. Intake of bacitracin and vancomycin had pronounced effects on faecal SCFAs excretion and reduced median total concentration of SCFAs from 105.4 mmol/kg to 21.8 mmol/kg and from 69.3 mmol/kg to 19.4 mmol/kg, respectively (p less than 0.05). Erythromycin had moderate effects on the faecal SCFAs excretion, whereas small or no changes were seen during intake of co-trimoxazol, doxycycline, nalidixic acid, and ofloxacin. 2-Methylbutyric acid, a SCFA not previously seen in human faeces, was found in the faeces of all subjects (median concentration before intake of antibiotic, 1.3 mmol/kg). Bacitracin, erythromycin, nalidixic acid, and vancomycin were detected in high concentrations in faeces during therapy, whereas trimethoprim, doxycycline, and ofloxacin were found in relatively low concentrations. In conclusion, some, but not all, peroral antimicrobials induce changes in faecal SCFAs, most likely reflecting changes in the colonic ecosystem.