Two strains of mice, Swiss Webster and DBA/2Cr, were injected intraperitoneally or intravenously with varying dosages of Fusobacterium necrophorum. The ability to eliminate the infection was assessed by quantitative enumeration of the organisms present in the blood, liver, and spleen, Three- to 4-week-old DBA/2Cr mice were highly resistant to both routes of injection. The intraperitoneal injection of older mice failed to demonstrate a dose-effect relationship whereas an intravenous injection of as few as 10(4) cells of F. necrophorum produced progressively necrotic leg abscesses, apparently involving the lymphonodus ischiadicus which filters the site of injection. Mortality was increased with sensitization by a previous sublethal injection. Also, an ethanol-killed cell vaccine delayed the onset of lethal infection, whereas repeated sublethal live cell injections provided nonspecific protection since mice vaccinated with the growth medium were equally protected. The development of leg abscesses after intravenous injection visibly demonstrated the pathogenicity of F. necrophorum and may provide a suitable model for the evaluation of vaccines and the effectiveness of antibiotics.