Brucella abortus contains a protein that elicits an antigenic response in cattle previously exposed to the organism. The amino acid sequence of the recombinant form of this antigenic protein was determined by gas-phase sequencing of the pyridylethylated protein and its peptides obtained by digestion with cyanogen bromide (CNBr), clostripain, and Staphylococcus aureus V8 protease. The Brucella protein demonstrated 53.6% identity with the Cu-Zn superoxide dismutase (SOD) from Photobacterium leiognathi. Residues essential for metal coordination and enzymatic activity and cysteines required for the formation of the intrasubunit disulfide bridge of Cu-Zn SOD were conserved in the Brucella protein. also exhibited SOD activity that was inhibited by cyanide, which is characteristic of a Cu-Zn SOD. Brucella abortus Cu-Zn SOD is the second prokaryotic Cu-Zn SOD to be sequenced, and the fifth found in prokaryotes. The high degree of conservation between Photobacterium and Brucella Cu-Zn SOD supports the hypothesis of a separately evolved prokaryotic and eukaryotic Cu-Zn SOD gene.