The development of acquired resistance to Brucella abortus was studied in mice infected intravenously with small numbers of live CO2-dependent B. abortus 544 used as vaccinal strain. CO2-independent B. abortus was used as challenge strain. The influence of several parameters in the development of acquired resistance was examined: vaccination--challenge interval, challenge route, dose of challenge, challenge-sacrifice interval. The behavior of the challenge strain in normal mice depended on the size of the inoculum. After an intravenous inoculum of about 1 X 10(6) bacteria, the level of infection in the liver or the spleen followed three phases: a phase of increase of the viable counts, a phase of decrease, and a phase of plateau. The decreasing phase was more pronounced in liver than in spleen. After an intraperitoneal injection of the same dose of bacteria, an infection developed following approximately the same pattern, except during the first day of infection. One month after vaccination by an inoculum of about 1 X 10(4) viable B. abortus, an accelerated appearance of the decreasing phase prevented almost completely the multiplication of an intravenous challenge of about 1 X 10(6) B. abortus in the spleens. In the livers the same phenomenon took place after a small increase of the bacterial population. After an intraperitoneal challenge, the numbers of Brucella isolated from the spleens and livers of vaccinated mice first fell sharply to practically nothing around the sixth day, and then rose to reach a phase of slow increase or of plateau until the twenty-fourth day. In our experimental conditions, the vaccine population remained undisturbed by the reinfecting inoculum.