Ostertagia ostertagi adult worm extracts were analysed by Western blotting using sera from calves experimentally infected with O. ostertagi and Cooperia oncophora. Strong differences in antigen recognition were noticed, even between animals from the same group. Two Ostertagia specific antigens with apparent molecular mass of 19.7 (OoA19.7) and 20.7 kDa (OoA20.7) were identified. One of them (OoA19.7) was purified by three subsequent chromatographic steps, i.e. gelfiltration, ion-exchange and reversed phased chromatography. It was demonstrated that this antigen does not show any cross-reactions with heterologous sera from C. oncophora, Dictyocaulus viviparus, Nematodirus helvetianus and Fasciola hepatica-infected animals. It was found that only IgG1 antibodies reacted against OoA19.7. The application of this antigen in an ELISA resulted in a highly species-specific test when compared to crude worm extracts. However, strong individual differences in anti-OoA19.7 response could be noticed between calves which received the same number of O. ostertagi larvae. These individual differences can hinder the application of the ELISA as a diagnostic tool, since the anti-OoA19.7 response does not seem to reflect the level of exposure to L3 larvae. This was supported by the absence of a clear infection-dose-related effect. It was shown that the anti-OoA19.7 response started from week 6 to 8, and reached its highest level at week 15.