Affinity-purified 38 kDa surface antigen of Neospora caninum tachyzoites was used to develop an enzyme-linked immunosorbent assay (ELISA) to diagnose N. caninum-associated abortion in cattle. The specificity of this antigen was demonstrated by using sera from cattle experimentally infected with other apicomplexan parasites. In a panel of field sera, serological differences could be demonstrated between herds with epidemic and endemic abortions. Optimal ELISA cut-offs were determined for the detection of association between seropositivity and abortion in herds with N. caninum-associated abortions. These optimal cut-offs differed markedly when only sera from either endemic or epidemic cases were used for cut-off determination. It may thus be appropriate to apply different cut-offs in serological tests depending on the abortion pattern to be analysed.