Cell-mediated immune (CMI) responses in swine naturally infected with Brucella suis biotype 3, swine suckling an infected sow and Brucella-noninfected swine were studied by an in vitro lymphocyte transformation procedure. The antigen used was a soluble antigen prepared from killed cells of B suis biotype 3. Lymphocytes were prepared from peripheral swine blood by the Ficoll-Hypaque technique, suspended in RPMI-1640 medium (1.5 X 10(6) lymphocytes/ml), cultured with B suis-soluble antigen or concanconcanavalin A, and incubated for 6 days. Sixteen hours prior to termination of incubation, cultures were labeled with 1 muCi of [3H]thymidine and, after harvesting, were assayed for [3H]thymidine incorporation into DNA by liquid scintillation spectrometry. Agglutination tests were conducted on sera collected simultaneously with samples for lymphocyte-stimulation tests. The B suis-soluble antigen elicited specific stimulation in lymphocytes from infected pigs. On a group basis, there was high correlation between the amount of serum antibodies and specific lymphocyte stimulation, but on an individual animal basis, there was little correlation of the results of both systems in infected swine. There was high correlation between recovery of Brucella from the tissues of animals and the degree of CMI response. Suckling pigs from an infected sow did not develop CMI responses, as measured by our system.