Three Brucella antigen preparations, Brucella abortus soluble antigen, B. abortus strain 45/20 enriched protein antigen, and B. melitensis enriched protein antigen, were compared in terms of their ability to induce specific in vitro lymphocyte immunostimulation responses. Lymphocytes were prepared from peripheral blood of cattle with different exposure experiences to B. abortus organisms. Lymphocytes were processed by the Ficoll-diatrizoate technique, and results were assayed for [3H]thymidine incorporation into DNA by liquid scintillation spectrometry. The three Brucella antigen preparations were compared both at their optimal concentrations of protein and on an equal-dry-weight basis. The results were evaluated in terms of specific lymphocyte immunostimulation responses induced by each preparation and the degree of correlation with infection. B. abortus soluble antigen-induced lymphocyte immunostimulation response correlated best with infection status followed by B. abortus 45/20 and B. melitensis enriched protein antigens. The implications of these findings are discussed and a hypothesis is proposed.