We compared the in-vitro activity of thiamphenicol against 100 strains of Gardnerella vaginalis with the activity of 11 other antimicrobial agents. The MICs for thiamphenicol ranged from 0.39 micrograms/ml to 6.25 micrograms/ml. The concentration at which 50% of strains were inhibited (MIC50) was 1.96 micrograms/ml, and the concentration at which 90% of strains were inhibited (MIC90) was 3.93 micrograms/ml. All strains were very susceptible to erythromycin, chloramphenicol, beta-lactam antibiotics, and clindamycin. Tetracycline and metronidazole were only moderately active. In an attempt to cure G. vaginalis-associated vaginitis with a single-dose treatment, we administered 2.25 g of thiamphenicol to 20 volunteers; 17 were clinically and bacteriologically cured. In two cases we observed that G. vaginalis was not eliminated immediately (i.e., at the first follow-up visit), but we saw a progressive disappearance of the strain without further treatment. In one case the treatment seemed to have failed but reinfection could not be ruled out. The results show that a single dose of thiamphenicol can cure G. vaginalis-associated vaginitis