The immunogenic and mitogenic properties of Brucella abortus 1119-3 bacterin (BA) and biologically active B. abortus lipopolysaccharide (BA-LPS) were studied using normal and athymic (nude) BALB/c and C3H/HeJ mice. Although BA stimulated 2-mercaptoethanol-sensitive (2-ME-S) primary and secondary antibody responses in all mice, nude mice, in contrast to normal BALB/c and C3H/ HeJ mice, did not make substantial 2-mercaptoethanol-resistant (2-ME-R) antibody responses. Similarly, all mice injected with BA-LPS made 2-ME-S primary responses, and the secondary response of thymus-bearing mice contained a substantial 2-ME-R component. Collectively, these observations suggest that although both BA and BA-LPS can stimulate thymus-independent 2-ME-S antibody synthesis, thymus-derived cells are required for optimal immune responses containing a 2-ME-R component. The antibody responses of normal BALB/c and C3H/HeJ mice to BA and BA-LPS were qualitatively and quantitatively similar. Both BA and BA-LPS were mitogenic for spleen cells from normal and nude BALB/c and C3H/HeJ mice but not for thymus cells from normal BALB/c or C3H/HeJ mice, suggesting that both preparations are B-cell mitogens.There are conflicting reports (5,12,16,21,37) about the ability of the brucellae to elicit the biological and immunological responses evoked by the Enterobacteriaceae. These early reports, based on various animal models, either confirmed or denied the cytotoxic, pyrogenic, and antigenic properties of killed Brucella abortus cells (BA) or B. abortus lipopolysaccharide (BA-LPS). The properties attributed to the BA-LPS were further clouded because different investigators have used products ofvarious purification procedures as the endotoxin preparation. In 1960, Redfearn (Ph.D. thesis, Univ. of Wisconsin, Madison, Wisc.) reported that, unlike enterobacterial LPS which partitions into the aqueous-phase, BA-LPS extracted by the Westphal procedure partitions into the phenol phase. Leong et al. (16) thoroughly documented that this phenol-phase-extracted BA-LPS shares many of the physiological, structural, and chemical properties of aqueous-phase-extracted enterobacterial LPS. Their results also indicated homology between BA-LPS and Escherichia coli LPS.Although many immunological responses by animals to endotoxin preparations from the Enterobacteriaceae have been well characterized (7,9,15,24,27,35), investigation of the immu-nological properties of BA-LPS has been largely neglected. We have studied several aspects of the immunological response by mice and mitogenic responses of murine cell cultures to BA and BA-LPS in light of several known immunogenic characteristics of E. coli bacterin and LPS. Congenitally athymic (nude) mice were used to investigate the role of T lymphocytes in the response to both BA and BA-LPS. C3H/ HeJ mice, shown to have a genetically controlled low response to the endotoxins of E. coli and Salmonella (27,30,32,36) were included in these studies to determine if this genetic defect would also limit the C3H/HeJ response to BA ...