The ability of aerobic and facultatively anaerobic endocervical flora to inhibit the growth of Neisseria gonorrhoeae in vitro was assayed. Factors influencing the occurrence of inhibitory components of the flora in vivo were evaluated. Endocervical swabs were obtained from 229 women at a local venereal disease clinic. Endocervical flora and N. gonorrhoeae were isolated and identified, and the ability of the flora to inhibit the growth of N. gonorrhoeae was determined by an agar overlay assay. Results revealed the most active inhibitors to be streptococci, staphylococci, and lactobacilli, in that order. Among only those women harboring inhibitory endocervical flora, inhibitory lactobacilli were recovered from fewer women infected with N. gonorrhoeae than uninfected women (P < 0.05). Among women having contact with an infected partner, those who subsequently developed gonorrhea were less likely to have inhibitory lactobacilli than those who did not become infected (P < 0.05). No other significant differences in the composition of the inhibitory flora were noted between infected and uninfected women. During the 2 weeks following menses, recovery of inhibitory lactobacilli on culture was highest, whereas recovery of N. gonorrhoeae was lowest. These observations suggest that the presence of certain lactobacilli may reduce risk of acquisition of N. gonorrhoeae following exposure to infected partners and that the potential protective effect may be greatest during the 2 weeks after menses.