Seventy-nine suspected false-positive sera, obtained over 1 year from routine submissions forBrucella ovisserological testing, were used in this study. These sera, which exhibited titres in the complement fixation test, but which because of their epidemiological history and their reactions in the enzyme-linked immunosorbent assay and gel diffusion test wereisuspected to be false positives, were further analysed by immunoblotting. In blots, usingB. ovisantigens, rough lipopolysacchride was identified as the major, immuno-reactive bacterial component. Antibodies against this macromolecule were present in 46.8% of the suspected falsepositive sera.