Sixteen patients with acute meningitis caused by Gram-negative bacteria were treated with pefloxacin intravenously. The age range of the patient group was six months to 85 years with a mean age of 40 years; three patients were children. In all but two patients meningitis was a complication of neurosurgical operations and fourteen of the sixteen had received prior therapy which was not successful. The causative organisms were: Pseudomonas aeruginosa (5), Acinetobacter calcoaceticus (4), Klebsiella pneumoniae (3), Enterobacter cloacae (2), Citrobacter diversus (1) and Salmonella group C (1). Pefloxacin was administered intravenously 800 mg twice a day to the adult patients (mean dosage of 21( +/- 6.7) mg/kg body weight) for a mean period ( +/- S.D.) of 11( +/- 4) days. The mean cerebrospinal fluid concentration of pefloxacin was 8.8( +/- 5.0) mg/l which was 54% of the mean peak serum concentration (16.3( +/- 8.8]. The mean MIC and MBC of the causative organisms were 1.1( +/- 1.2) mg/l and 1.64( +/- 1.2) mg/l. Thirteen patients (87%) were cured or clinically improved and twelve (80%) were bacteriologically cured. One patient failed, another patient had reinfection and one was not assessable. No side effects were observed. In the present study pefloxacin offered an efficacious and safe treatment of Gram-negative meningitis following failure of other antibiotic therapy.