Twelve healthy male subjects age range 24-40 y participated in the investigation. The trial was divided into 2 35-d periods. The 2 treatment regimens were: (i) 1 x 400 mg moxifloxacin tablet in the morning and 1 placebo tablet in the evening for 7 d; and (ii) 1 x 500 mg clarithromycin tablet in the morning and 1 x 500 mg clarithromycin tablet in the evening for 7 d. Each subject received firstly I treatment regimen and secondly the other treatment regimen. The wash-out period was 6 weeks between the two treatment regimens. Moxifloxacin caused significant decreases of enterococci and enterobacteria during the administration period while the numbers of staphylococci, streptococci, Bacillus and Candida were not affected. No impact on peptostreptococci, lactobacilli, Veillonella, Bacteroides or fusobacteria was observed, while bifidobacteria and clostridia decreased during moxifloxacin administration. The microflora was normalized after 35 d. Clarithromycin caused significant reduction of Escherichia coli while the numbers of enterococci, Enterobacter, Citrobacter, Klebsiella and Pseudomonas increased markedly. No significant changes in the numbers of staphylococci, streptococci, Bacillus and Candida were noticed. In the anaerobic microflora bifidobacteria, lactobacilli and clostridia were suppressed, while no changes in peptostreptococci, Veillonella, Bacteroides and fusobacteria were found. The microflora was normalized in all volunteers after 35 d.