The roles of humoral and cell mediated immune responses in murine brucellosis were investigated in this study.B. abortus strain 19, the current vaccine strain, is known to induce an antibody as well as cell mediated immune responses, both of which protect mice against smooth strain 2308. B. abortus rough strain RB51 does not induce an o-side chain specific antibody response and yet protects mice against smooth strain 2308. Passive transfer experiments using serum and nylon wool enriched T cells obtained from micevaccinated with strain 19 and strain RB51 were carried out. Immune senum from strain 19 vaccinated mice protected against challenge with strain 2308 but not strain RB51. Nylon wool enriched T cells from strain 19 vaccinated mice protected recipient mice against challenge with both strains RB51 or 2308. Serum obtained from RB51 vaccinated mice did not protect recipient mice against challenge with either strain RB51 or strain 2308. Nylon wool enriched T cells from the same vaccinated mice, however, protected mice against challenge with both strains. Thioglycollate elicited mouse peritoneal macrophages could be activated by recombinant gamma-interferon to kill ingested B. abortus. This was true for both the rough strain RB51 and smooth strain 2308, although RB51 exhibited greater susceptibility to killing. Macrophages already invaded by either strain RB51 or strain 2308 retained their responsiveness to gamma-interferon activation am could kill either strain of B. abortus following activation by gamma-interferon. Results obtained in this investigation indicate that strain RB51 protects mice against strain 2308 by probably inducing a cell mediated immune response. / Ph. D.