Imipenem was evaluated for its in vitro and in vivo activities alone and in combination with gentamicin against a clinical isolate of Listeria monocytogenes, and the results were compared with the activities of ampicillin with and without gentamicin. In vitro, the MBC of imipenem was fourfold less than that of ampicillin. Checkerboard determinations of the MBCs exhibited a synergistic response for imipenem-gentamicin but an indifferent response for ampicillin-gentamicin. In vivo studies with experimental bacteremia and meningitis due to L. monocytogenes in newborn rats revealed that both imipenem-cilastatin and ampicillin at a dose of 50 mg/kg produced excellent bactericidal titers in serum. Overall mortality rates were not significantly different among four groups of animals receiving imipenem-cilastatin, imipenem-cilastatin-gentamicin, ampicillin or ampicillin-gentamicin. However, imipenem-cilastatin alone or in combination with gentamicin was significantly less effective than ampicillin-gentamicin, as judged by the rapidity of clearance of bacteria from blood, liver, and spleen. These findings suggest that imipenem-cilastatin and imipenem-cilastatin-gentamicin may not be suitable alternatives for the treatment of listeriosis.