The activity of rifampicin was compared with that of tetracycline, ampicillin, penicillin G, chloramphenicol, gentamicin and sulphadiazine in vitro and in experiments on mice infected with various strains of Listeria monocytogenes. Although the minimum inhibitory concentration of rifampicin was not appreciably lower than that of ampicillin, penicillin G and gentamicin, rifampacin was very much more active in vivo than any of these antibiotics. One of the reasons for its greater efficacy may be that it inhibits or destroys intracellular Listeria. In addition, a synergistic effect was observed in vivo when rifampicin was administered in combination with ampicillin or tetracycline. The combined administration of rifampicin with either of these antibiotics should prevent the emergence of rifampicin-resistant Listeria.