OBJECTIVE The aims of this study were to examine the effect of the use of a mouthwash containing 0.05% cetylpyridinium chloride (CPC) on (1) the composition of the normal oral microbial flora and (2) the establishment of non-oral bacterial species in the mouth. METHODS In a double-blind study, 129 subjects used, twice daily for 6 weeks, either a randomly assigned test or placebo mouthwash. At baseline and at the completion of the study, a sample of the oral flora was obtained by asking each subject to "swish" around the mouth for 30 s, 10 ml of distilled water. The numbers of total colony forming units (CFU), facultative bacteria, streptococci, mutans streptococci, lactobacilli, staphylococci, yeasts and enterococci were enumerated after culture on eight non-selective and selective media. RESULTS After 6 weeks use of either the test or placebo mouthwashes, there were no significant differences (analysis of variance) between the counts (CFU/ml of sample) for any of the microbial taxa. CONCLUSIONS These data indicate that the use of a CPC-containing mouthwash does not alter the composition of the normal oral flora or result in the establishment of non-oral and potentially pathogenic bacteria in the mouth.