cDNA encoding an immunogenic protein from partially sporulated oocysts of Eimeria acervulina was cloned and used to search for the homologous counterpart in Eimeria tenella. Monospecific antibodies were raised against the recombinant expression product. Using these antibodies, the parasite proteins were found to be localised in the refractile bodies. The derived amino-acid sequences were compared by computer using the SWISSPROT protein database. In addition to high homology between the Eimeria species, extensive similarity was found with pyridine nucleotide transhydrogenase from Escherichia coli. Comparison with the sugar signature database also resulted in a possible sugar binding domain present only in the Eimeria proteins. It is possible that the corresponding parasite proteins play a role in the recently discovered mannitol cycle of Eimeria.