Brucella abortus st. RB51 is a rough mutant of smooth st. 2308 devoid of O-side chain and resistant to rifampin. The purpose of this investigation was to study the behavior and effects of viable st. RB51 organisms in inoculated mice and cattle and to further substantiate the lack of O-side chain antigens in this strain. A single injection of live st. RB51 persisted in BALB/C mice up to 28 days. A secondary exposure was cleared in 7-21 days. One or 2 injections of st. RB51 did not induce detectable titers of anti-O-side chain antibodies, although antibody titers to st. RB51 whole cell and cytoplasmic antigens were detected. Mice infected with st. RB51 alone or followed by infection with st. 2308, demonstrated a very strong reaction to a 14-18 Kd antigen which was believed to be the core of the LPS complex. When st. RB51 was administered after injection of st. 2308 the response to the core determinants were inhibited. One vaccination with st. RB51 was able to significantly protect mice against challenge with st. 2308 at one and four weeks post challenge. Two st. RB51 vaccinations were able to protect mice as well as one vaccination at one week post challenge but protection increased by four weeks post challenge. Strain RB5l was able to survive in cattle a for at least twenty-two days. The organism remained stable, rifampin resistant, and may have induced minor amounts of transient anti-O-side chain antibodies in some cows late in the experiments. / Master of Science