Antibiotic susceptibility and synergy were studied in 12 clinical isolates of Listeria monocytogenes from patients with meningitis and septicemia. Rifampin and trimethoprim-sulfamethoxazole (TMP-SMX) were the most potent single drugs tested. Approximately 80% of the strains demonstrated full synergistic bactericidal activity with rifampin in combination with penicillin or ampicillin. Clinical experience dictates that ampicillin or penicillin should remain the antibiotic of choice in the treatment of severe infections, such as meningitis caused by L. monocytogenes. Where the use of penicillin is contraindicated (e.g., allergy or failure to respond), use of TMP-SMX might be considered. Further in vitro and vivo studies are needed before therapy with rifampin or TMP-SMX in combination with penicillin or ampicillin can be recommended.