11 volunteers received 1 g ceftriaxone i.m. every 24 h for 5 consecutive days. Volunteers were selected on the basis of the "ceftriaxone inactivating capacity" of their faeces, which should be 250 mg/kg faeces. The effect of treatment on the aerobic oral and faecal flora was studied. In the oral flora a temporary suppression of the viridans streptococci occurred, which normalised already during treatment. More spectacular was the effect on the faecal flora: suppression of aerobic Gram-negative bacilli with overgrowth of yeasts and enterococci. Two weeks after treatment the results of the faecal cultures were almost the same as before treatment in most of the volunteers. One volunteer experienced severe diarrhoea, starting the day after completion of ceftriaxone treatment and lasting 2 days.