The extended-spectrum penicillins ticarcillin, mezlocillin, and piperacillin might be useful as single agents for the treatment of pelvic infections in women if it could be shown that these drugs are active against Chlamydia trachomatis. We found that the MIC90 (concentration at which 90% of strains are inhibited) values of ticarcillin, mezlocillin, and piperacillin were 16, 16, and 64 micrograms/ml, respectively. Several cephalosporins were found to have MICs for C. trachomatis of greater than 200 micrograms/ml. Ten women with postpartum endometritis who were colonized with C. trachomatis had repeated chlamydial cultures following treatment with beta-lactam antibiotics. All seven cases treated with ticarcillin plus clavulanic acid (a beta-lactamase inhibitor) or piperacillin alone had C. trachomatis-negative cultures after treatment. Three of four of these women had negative cultures at a second follow-up visit. In contrast, the three women treated with a cephalosporin were culture-positive at their first follow-up clinic visit. These data suggest that there is a correlation between the in-vitro measurement of beta-lactam antibiotic activity against C. trachomatis and the microbiologic outcome of treatment. We conclude that the extended-spectrum penicillins deserve further evaluation as single agents for the treatment of pelvic infections in women at high risk for C. trachomatis infections.