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Pasteurella multocida is a commensal of the upper respiratory tract of various animals. The spectrum of human diseases caused by P. multocida varies from soft-tissue infections fol- lowing bites and scratches to systemic infections, including res- piratory tract infections resulting from airborne contamination and/or chronic carriage. Antimicrobial resistance of Pasteurella strains originating from animals has been reported for many years. Human P. multocida isolates are usually susceptible to penicillins. We report a case in which a strain of b-lacta- mase-producing P. multocida was isolated from a lung abscess specimen and review related cases. A 75-year-old woman with chronic bronchitis was hospital- ized with fever (temperature, 407C), weight loss, increased pro- ductive cough, and dyspnea. A chest x-ray was unremarkable, whereas the WBC count was 13,900/mm 3 (89% neutrophils) and the one-hour erythrocyte sedimentation rate was 63 mm/h. The patient was treated for 8 days with intravenous amoxicillin (1.5 g/d). Because of persistent signs of infection, a new chest ra- diograph was obtained, which showed a left lung abscess; the abscess was confirmed by CT. Culture of specimens obtained by bronchoalveolar lavage yielded P. multocida subspecies mul- tocida (10 5 cfu/mL) and Haemophilus parainfluenzae (10 3 cfu/ mL). Production of b-lactamase by both isolates was detected by a chromogenic test using nitrocefin-impregnated disk and the synergic effect between clavulanate and amoxicillin by disk dif- fusion testing. Minimum inhibitory concentrations of amoxi- cillin and amoxicillin/clavulanate for the P. multocida strain were of 8 and 0.25 mg/mL, respectively. Consequently, amoxi- cillin was changed to intravenous amoxicillin/clavulanic acid (3 g/d). A favorable outcome was achieved, and the patient was discharged from the hospital; medication at the time of dis- charge was oral amoxicillin/clavulanic acid for 2 months. Our patient had no known exposure to animals other than her cat. Two different strains of P. multocida (subspecies multocida and septica) were isolated from the cat's saliva, but none of them was found to produce b-lactamase. A MEDLINE search of the literature revealed only 4 well- documented cases of human infections due to penicillin-resis- tant P. multocida (1-4). All 4 cases were respiratory tract in- fections. The principal features of these cases along with our case are presented in table 1. The respiratory tract is the most common site of pasteurella infections after soft tissues (5). Pre- vious studies have shown that impaired pulmonary defenses