Cefotaxime, imipenem, gentamicin, and doxycycline were active in vitro against the virulent serotype O8 Yersinia enterocolitica WA strain. Amoxycillin was inactive. The in vivo activity of these drugs was evaluated in a standardized and reproducible mouse model of systemic infection. Each single antibiotic was injected intravenously 30 h after intravenous inoculation of Y. enterocolitica WA. In vivo efficacy was measured by counting the viable bacteria recovered from the whole spleens of mice sacrificed at selected times. Doxycycline and gentamicin were active in stopping bacterial proliferation. Cefotaxime and imipenem, even at high doses (250 and 100 mg/kg of body weight, respectively), were totally ineffective, as was amoxycillin. Bacterial inocula (10(7)), recovered from either the in vitro growth or the infected spleens, were plated on cefotaxime or imipenem concentration gradients in agar; no emergence of beta-lactam-resistant organisms was detected. Based on these experiments it is not possible to explain, from any given property of the antibiotic, the bacteria, or the host, the discrepancy between the in vivo and in vitro activities of cefotaxime and imipenem. On the basis of these results, the use of newer beta-lactam antibiotics should be delayed in the therapy of human Y. enterocolitica infections until further investigations are carried out.