Rifampin, a broad-spectrum antibiotic able to penetrate intracellularly, was used for treatment of infections with Brucella melitensis in mice and Brucella abortus in guinea pigs. Treatments were administered for seven, 14, or 21 days; mice were given 25 mg of rifampin/kg per day, and guinea pigs 100 mg/kg per day. Efficacy of the drug was determined by comparison of rifampin-treated animals with saline-treated controls and with tetracycline-treated mice (200 mg/kg per day) according to the following criteria: (1) primary infections of the spleen and (in guinea pigs) of the lymph nodes; (2) residual infections of the spleen, i.e., infections shown after complementary treatment with suspensions of killed Corynebacterium parvum or with cortisone; (3) splenomegaly; and (4) serological response (in guinea pigs). Treatment with rifampin, even for one or two weeks, drastically reduced the number of infections by all of these criteris, and treatment for three weeks cured nearly all mice; the incidences of primary and residual infections in rifampin-treated mice after three weeks were 0 and 8.5%, respectively, as compared with 70.3% and 73.5%, respectively, in tetracycline-treated mice. Of 25 guinea pigs treated with rifampin for three weeks, spleen infection was shown in one, and lymph node infections in 10.