The effect on the faecal flora of cefuroxime axetil, an oral ester of cefuroxime, was examined in ten healthy volunteers following a dose of 250 mg twice a day for 41/2 days. The mean blood concentration 2 h after the ninth dose was 3.6 mg/l and the urinary excretion was approximately 40% in 6 h and 50% in 24 h. Two volunteers developed mild diarrhoea and one candida vaginitis. Although there was variation between volunteers in the changes found in the faecal flora, the major effects were a reduction in the total anaerobic bacterial count, particularly where high levels of cefuroxime were present in the faeces and a significant decrease or elimination of strains of Enterobacteriaceae plus an increase in the streptococcal count. Six volunteers showed an increase in the candida count. Clostridium difficile was not isolated from any volunteer and toxin was not present in the two volunteers who developed diarrhoea. Four volunteers showed high levels of cefuroxime in the faeces on the fourth or fifth days of treatment.