The diffusion of moxalactam into cerebrospinal fluid was studied in 18 hospital patients with purulent meningitis treated with this antibiotic. Moxalactam was administered by intravenous infusion in doses of 2 g 8-hourly (6 g/24 h). Treatment lasted from 10 to 30 days depending on the pathogen involved. The antibiotic was assayed by high performance liquid chromatography simultaneously in serum and CSF at the time of lumbar puncture. Depending on the time elapsed since the end of the infusion, the mean CSF concentrations ranged from 32.8 to 9.37 micrograms/ml at the beginning of the disease and from 23.75 to 3.78 micrograms/ml toward the end of treatment. These values were notably higher than the MIC for most of the micro-organisms encountered. All patients were cured.