Eight isolates of Brucella abortus were obtained from cows before and after they were treated with oxytetracycline and streptomycin. The susceptibility to these antibiotics was determined by broth-dilution minimal inhibitory and minimal lethal concentrations. Differences were not found in the minimal lethal concentrations of oxytetracycline or streptomycin in isolates obtained from cows before and after they were treated. This indicates that treatment failures in the cows were not the result of development of resistance to the antibiotics by B abortus.