An ELISA was established to measure bovine IgG directed against the recombinant antigenic determinant of Nc-p43, a major surface antigen of Neospora caninum. In a previous study, two thirds of the C-terminal of the molecule was expressed as a $6{;}{ imes}{;}His$ tagged protein (Ncp43p) for ELISA using 2/3 of the N-terminal of SAG1 from Toxoplasma gondii as a control (TgSAG1A). Among 852 cattle sera collected from stock farms scattered nation-wide, 103 sera (12.1％) were found to react with Ncp43p positively, but no positive reaction was observed with TgSAG1A. This study shows that Ncp43p could be available as an efficient antigen for the diagnosis of neosporosis in cattle. Furthermore, it together with TgSAG1A, could be useful for the differential diagnosis of N. caninum and T.gondii infections in other mammals.