Brucella abortus proteins from virulent S2308 expressed from a pBluescript II SK- genomic library stimulated peripheral blood mononuclear (PBM) cell proliferation from cattle vaccinated with B. abortus S19. The method described here permits a rapid and directed approach to isolate genes encoding antigens of B. abortus that interact with lymphocytes primed to the living bacterium. The supernatants from the bacterial host JM109 (DE3) were cultured with freshly isolated bovine PBM cells. A total of 300 clones were evaluated. Ten clones were identified that stimulated T-lymphocyte proliferation. Among them, one clone with a 2.5-kb insert stimulated T-lymphocyte proliferation in all three animals, suggesting that the proteins encoded by genes within this fragment may represent immunodominant antigens. DNA sequencing of this clone reveals two large open reading frames (ORFs). ORF II has a high degree of similarity to the Escherichia coli ssb gene, which codes for the single-stranded DNA binding protein. ORF I, in the opposite direction to ORF II, shows similarity to the N terminus of the E. coli uvrA gene, which codes for one of the three subunits of the E. coli ABC excision nuclease. The observation that the PBM cells recognized and proliferated in response to proteins expressed from single clones provides a novel strategy to select bacterial antigens that may prove useful in designing alternative vaccines against brucellosis.