The adherence of the normal flora to human uroepithelial cells was examined using scanning and transmission electron microscopy and an in vitro adhesion assay. The normal flora were isolated from human cervical, vaginal and distal urethral surfaces. Human uroepithelial cells were obtained from healthy females (controls) and from female patients with recurrent urinary tract infection. The results indicate that: 1) there was no significant difference between the mean adherence of the normal flora to the uroepithelial cells from controls or patients, but there were quantitative differences between different bacterial species of the normal flora attaching to uroepithelial cells; 2) bacterial species isolated from the distal urethral and vaginal surfaces attached in larger numbers to uroepithelial cells from controls than from patients at day 10 of the menstrual cycle (peak value of adherence); 3) the adherence patterns of the cervical and vaginal microbial flora were bicyclic and appeared to be related to the human menstrual cycle; 4) complete or partial inhibition of adherence of several Gram-negative uropathogens was achieved by preincubating the uroepithelial cells with several bacterial species of the normal flora. Results from this study suggest that the normal flora of the urinary tract may play an important protective role against attachment of uropathogens to the surfaces of uroepithelial cells, and that the blocking capacity of these organisms may vary over the menstrual cycle.