One hundred eighty-four postpartum patients with the diagnosis of endomyometritis underwent intrauterine culturing. Beta-Lactamase production was tested using nitrocephin-impregnated discs in 278 bacterial isolates from patients with postpartum endometritis. beta-Lactamase production was found to be present in 149 of 278 (54%) isolates tested: 0 of 39 Streptococcus faecalis (0%), 22 of 41 Escherichia coli (54%), 66 of 68 Bacteroides bivius (97%), 0 of 13 Proteus mirabilis (0%) and 15 of 42 Staphylococcus epidermidis (36%). Anaerobes, as a group, had the highest rate of beta-lactamase production, 94 of 102 (93%), followed by gram negatives, 34 of 84 (40%), then gram positives, 21 of 91 (23%). However, it was the beta-lactamase produced by the gram-negative bacteria that was most responsive to the addition of a beta-lactamase inhibitor. The frequent isolation of beta-lactamase-producing bacteria from patients with postpartum endometritis raises concern about the use of antibiotics not resistant to these enzymes. Therefore, antibiotics combined with a beta-lactamase inhibitor may have potential as single-agent therapy in obstetric and gynecologic infections.