The in vitro susceptibilities of 45 recent clinical isolates of Chlamydia trachomatis obtained from women with asymptomatic genital tract infection, mucopurulent cervicitis, or pelvic inflammatory disease to doxycycline, azithromycin, ofloxacin, and clindamycin were determined. In addition, susceptibilities of 12 isolates to amoxicillin and trimethoprim-sulfamethoxazole were also determined. Isolates also were serotyped with a panel of monoclonal antibodies specific for chlamydial major outer membrane protein; 24 of 45 (53%) belonged to serovars Ia and E. For all isolates, the MIC range of doxycycline was 0.008 to 0.06 micrograms/ml, for trimethoprim-sulfamethoxazole it was 0.03 to 0.25 micrograms/ml, for azithromycin it was 0.125 to 2.0 micrograms/ml, for ofloxacin it was 0.5 to 1.0 micrograms/ml, for clindamycin it was 0.25 to 2.0 micrograms/ml, and for amoxicillin it was 0.25 to 4.0 microgram/ml. The ranges of minimum chlamydiacidal concentrations were generally 1 to 4 dilutions above the MICs of most agents, with a rank order similar to those of the MICs. Comparing the minimum chlamydiacidal concentrations for 90% of isolates tested, isolates causing asymptomatic infection belonged to a greater variety of serovars and were relatively more susceptible to doxycycline and azithromycin than isolates causing mucopurulent cervicitis or pelvic inflammatory disease; these differences in susceptibility were not detected among the other study agents. These data indicate that additional studies are needed to better define the apparent association of certain chlamydial serovars with the clinical severity of disease and the in vitro susceptibilities to certain antimicrobial agents.