In 1987 we reviewed the literature on the concept of colonization resistance (CR)(12). In this concept, the indige-nous anaerobic flora limits the concentration of potentially pathogenic (mostly aerobic) flora in the digestive tract. This implies that the risk of superinfections by aerobic flora would be eliminated by selecting those antimicrobial agents which spare the anaerobic flora. The concept of CR was based on experiments in animals and uncontrolled observations in pa-tients. It was not validated conclusively. Since publication of our previous review (12), we have improved the design for studying the influence of antimicrobial agents on CR, especially by using human volunteers and analyzing the data in each volunteer separately with singlepatient statistics (57, 63). This methodology was applied in the study of several antimicrobial agents (57-61, 63, 64). The resulting data provide a better understanding of the impact of antimicrobial agents on the microbial flora of the bowel. At present, it seems to be the case that some former conclusions were made prematurely, especially those pertaining to the availability of antimicrobial agents that do not impair CR (12, 47).