To test the efficacy of rough Brucella strain vaccines in sheep, a vaccine recently developed in cattle (Brucella abortus strain RB51) was assessed in comparison with the conventional Rev. 1 vaccine. Forty-five ewes from twelve to fourteen months of age, from brucellosis-free flocks, were allotted to three groups of fifteen ewes each. Group one was vaccinated by the conjunctival route with 1.73 x 10(8) colony forming units (CFU) of Rev. 1 vaccine. Group two was vaccinated subcutaneously with 11 x 10(9) CFU of RB51 vaccine and group three was considered as a control. All sheep were challenged at two to three months of gestation with 5 x 10(7) CFU of virulent B. melitensis H38. Vaccination with RB51 vaccine did not result in the production of any antibodies against the O-side chain of lipopolysaccharide, as measured by conventional serological tests (Rose Bengal plate test and complement fixation test). Protection of sheep against abortion and excretion of virulent Brucella strain in vaginal fluid, aborted foetuses and/or non viable lambs at parturition and abortion was significantly lower than that afforded by Rev. 1 vaccine. The difference compared to the control group was not significant. Data from this study suggest that the RB51 vaccine used for cattle vaccination does not provide effective protection of sheep against abortion induced by B. melitensis.