Patients with purulent meningitis received amoxicillin-clavulanic acid (200/20 mg/kg [body weight] per day). Clavulanic acid levels in cerebrospinal fluid were less than or equal to 0.05 micrograms/ml in 5 of 18 samples and ranged from 0.1 to 0.8 micrograms/ml in the others. Of 12 cerebrospinal fluid samples tested, 10 lacked bactericidal activity in vitro against a beta-lactamase-producing strain of Haemophilus influenzae.