The sensitivities of three commercial peroxidase conjugates (polyclonal anti-sheep IgG, recombinant protein G and a monoclonal anti-ruminant IgG1) in an enzyme-linked immunosorbent assay for Brucella ovis were evaluated. The monoclonal and protein G conjugates reduced, but did not totally remove, the characteristic background reactivity observed with the sera from brucella-free sheep. The protein G conjugate provided the best sensitivity, similar to that obtained with the classical gel diffusion test. Both tests were highly specific when testing sera from brucella-free animals but detected as positive a large proportion of sera from both Brucella melitensis-infected and B melitensis Rev 1-vaccinated sheep.