Brucellosis caused by Brucella abortus is an important zoonotic disease characterized in cattle by placentitis, fetal death and abortion. Brucellosis research employing the pregnant mouse model has been limited to the bacteriology and immunology of the disease. Studies reported in this dissertation characterized the pathology and serology of one virulent (2308) and two attenuated (19 and RB51) strains of B. abortus in the pregnant mouse. When 10^5.7 strain 2308 organisms were administered intraperitoneally to BALB/c mice in midgestation, by 9 days post-inoculation they consistently produced a severe, necro-suppurative placentitis frequently asociated with fetal death. Sequential examination of the placenta during the second half of gestation revealed infection of the trophoblast giant cells and visceral yolk sac. As occurs in the cow, brucellae localized within the rough endoplasmic reticulum of the trophoblast cells. An inoculum of 10^7.5 strain 19, the current vaccinal strain, produced a similar lesion. Both strains 2308 and 19 induced antibodies against lipopolysaccharide O-side-chain as monitored by serum agglutination and western blot analysis. Inoculation of ^9.5 strain RB51 brucellae, a stable rough organism, produced minimal placentitis and did not induce fetal death or anti-O-side-chain antibodies as assessed by serum agglutination or Western Blot analysis. Mice vaccinated prebreeding with 10⁶ strain 19 demonstrated excellent protection against midgestational challenge by 10^5.7 strain 2308 organisms as assessed by lesion development and infection of spleens and placentas. Vaccination with 10⁸ strain RB51 produced a lesser, although significant, degree of protection against infection and prevented severe lesions and fetal death: Two inoculations of strain RB51 improved protection against placental infection. The intravenous transfer of strain 19 antiserum or monoclonal antibodies directed against the O-side-chain one hour before challenge with brucellae provided protective immunity to the pregnant mouse. In contrast, antiserum against strain RB51 was not protective, indicating the importance of cell-mediated rather than humoral immunity in protection afforded by vaccination with strain RB51. These experiments suggest that the pregnant mouse is an appropriate model to study the immunopathology of brucellosis. It also supports the development of strain RB51 as a vaccine for bovine brucellosis, offering protection against infection without eliciting anti-O-chain antibodies that confound the serodiagnosis of infection with virulent smooth strains. / Ph. D.