A lambda ZapII cDNA library was constructed using mRNA from Eimeria acervulina sporulated oocysts and screened with monoclonal antibodies raised against Eimeria tenella sporulated oocytes. Monoclonal antibody N3C8B12 identified a clone (6S2) potentially encoding an aspartyl proteinase since significant homology with cathepsin D, pepsin and renin proteinases was revealed by sequence comparisons. The 1500-bp cDNA fragment containing the coccidial gene was subcloned into pGEX-FA expression vector, leading to the production of an 80-kDa fusion protein (FA6S2) which was used to immunize rabbits. The anti-FA6S2 rabbit sera revealed a single 43-kDa protein present in Eimeria acervulina, Eimeria tenella, Eimeria maxima and Eimeria falciformis sporulated oocyst antigens. Indirect immunofluorescence and electron microscopy with mAb N3C8B12 localized the putative aspartyl proteinase in the refractile bodies of Eimeria tenella sporozoites.