IgG-antibody response in aborting sheep and in apparently healthy sheep in a flock against acid-glycine-extracted antigens from three strains for each C fetus subsp. fetus and C jejuni were analysed by Western blot. One strain of C fetus subsp. fetus was isolated from aborting sheep. Western blot analysis of the sera revealed the presence of IgG antibody binding to the common antigens including proteins with the Mw of 63 kDa and 54 kDa in extracts from both C. fetus subsp. fetus and C. jejuni strains. In addition, IgG antibodies in sera from aborting sheep reacted more strongly with the antigens from C. fetus subsp. fetus strains with Mw of approximately 100, 95 and 86.5 kDa than those of apparently healthy sheep. The binding profile of the antibodies with these antigens appeared to be unique for each C. fetus subsp. fetus strain. On the other hand, IgG antibodies only in sera from aborting sheep recognized strongly the antigens of each C. fetus subsp. fetus strain at the Mw ranged from approximately 26 to 22 kDa. However, the antigenic components between 26 and 22 kDa were not detectable in coomassie blue stained gel and thought to have non-protein nature. These low molecular weight antigens of C. fetus subsp. fetus may be related to a recent infection in aborting sheep. These observations indicate that such species-specific antigens or conjugated protein antigens could be used for improving the specificity of the serological tests to detect C. fetus antibodies in sheep sera, and may be the candidates for subunit vaccines against ovine abortion.