Chlamydia trachomatis infections are a major cause of morbidity in the sexually active. While current therapy is usually effective, isolates demonstrating relative resistance to erythromycin or heterotypic resistance to erythromycin, tetracycline and their congeners have been described, establishing a need to continue to evaluate other antimicrobial agents for possible efficacy. In this study trovafloxacin, a compound related to the fluoroquinolones, was tested in tissue culture for in-vitro efficacy against 19 strains of C. trachomatis, including three strains known to exhibit heterotypic resistance. All strains were fully sensitive to trovafloxacin with a minimum inhibitory concentration at which 90% of inclusions were reduced (IR90) of 0.05 +/- 0.07 mg/L (mean +/- S.D.). The IR90 for the comparison antibiotics erythromycin, doxycycline and ofloxacin were 0.17 +/- 0.07, 0.10 +/- 0.03 and 0.35 +/- 0.15 mg/L respectively. Trovafloxacin is very active on a weight basis and deserves further evaluation.