The vaginal pH, microbial flora and presence of clue cells were investigated in 89 women who were seen at a sexually transmitted diseases clinic with a vaginal discharge or because they were contacts of men with gonococcal or non-gonococcal urethritis or because they were seeking a routine examination. None of the women had received antibiotics for at least 4 weeks before examination. A clinically normal vaginal secretion was found in 21 (55%) of 38 women who had a vaginal pH of 5.0-5.5, while such a secretion was found in only 7 (14%) of 51 women who had a pH value of 6.0-7.5. Clue cells, Chlamydia trachomatis and Mycoplasma hominis were found two to three times more often in women with the higher pH value than in those with the lower value and Ureaplasma urealyticum and Trichomonas vaginalis also occurred more frequently in the former group. Furthermore, large numbers of M. hominis organisms (greater than or equal to 10(6) colour changing units/ml) were associated significantly with the higher pH value. However, there was no appreciable difference in the distribution of Candida albicans between the two groups. C. trachomatis but not the other micro-organisms was isolated most often from women who were taking oral contraceptives. The results indicate that a pH of greater than or equal to 6.0 is strongly predictive of infection and may be more useful than the type of discharge in suggesting a need for confirmatory microbiological tests.