The effect on the faecal aerobic and anaerobic flora of ciprofloxacin given in low doses for selective decontamination of the digestive tract was investigated in ten human volunteers. The volunteers received 50, 100 and 200 mg of ciprofloxacin every 12 h for five days at intervals of three and five weeks respectively. No significant differences in the numbers of aerobes or anaerobes were seen after the 2 x 50 mg regime. The colony counts of most anaerobes and the total aerobe count were significantly decreased after the 2 x 100 mg [corrected] regime. Whereas the aerobe count was also significantly decreased after administration of 2 x 200 mg, the anaerobe count remained stable. Clostridium difficile was not detected during or after treatment. From these results it can be concluded that ciprofloxacin in a dose of 2 x 200 mg [corrected] can be recommended for selective decontamination of the digestive tract.