Free radicals in the nature are harmful to organisms because they cause damage to tissues, cell membranes, proteins, and DNA. The animal immune system (cell mediated immune response) uses oxygen molecules to kill foreign organisms. B. abortus causes a cattle disease called brucellosis. The mechanism for bacteria to survive and adapt in the face of oxidative attack is thought to be important for understanding pathogenesis. This dissertation presents the response of B. abortus to oxidative stress and the regulation of catalase as a defensive mechanism;Different protein expression patterns were observed during oxidative stress by conducting 2-D protein gel analysis. DnaK (Hsp70) and GroEL (Hsp60) were not dramatically changed in the protein expression level, whereas, Cu-Zn SOD and catalase were upregulated in response to oxidative stress. Catalase is known as the critical enzyme for removal of hydrogen peroxide and the protection of cells from oxidative damage. I show that catalase is increased both in protein expression and in RNA transcription in the presence of hydrogen peroxide. To elucidate the regulatory mechanism, the regulatory protein (OxyR) of catalase transcription is identified. After cloning and sequencing Brucella oxyR (boxyR) gene, I found that boxyR is one of the LysR family of transcription factors with a helix-turn-helix DNA binding motif. I also show that bOxyR binds to the catalase gene promoter. Brucella oxyR is immediately upstream of the catalase gene but in the opposite orientation. The two genes share the same promoter region. The deduced amino acid sequence of boxyR shows only 40% identity with other known OxyR sequences. This is the first report that OxyR regulates catalase in the a subdivision of Proteobacteria (purple bacteria).