Ten healthy volunteers were given 200 mg cefpodoxime proxetil tablets every 12 h and ten volunteers received 500 mg amoxicillin tablets every 8 h for seven days and the impact of the agents on the oral and intestinal microflora was studied. In the oral microflora, only minor alterations were observed in both groups. In subjects receiving cefpodoxime proxetil, the numbers of streptococci, enterobacteria and clostridia were strongly reduced in the faecal flora, while there was an overgrowth of enterococci, yeasts and Clostridium difficile. Amoxicillin administration induced somewhat smaller alterations in the faecal microflora, although all subjects had overgrowth of new colonizing amoxicillin resistant microorganisms, mainly Escherichia coli, Klebsiella and Enterobacter. Beta-lactamase activity was detected in the flora of six volunteers from each group.