An Eimeria acervulina protein fraction was identified which conferred partial protection against an E. acervulina challenge infection. From this fraction a 37 kDa protein was purified and its corresponding cDNA was cloned and shown to encode a lactate dehydrogenase (LDH). Full length cDNAs encoding LDH from two related species, E. tenella and E. maxima, were also cloned. The homology between the primary amino acid sequences of these three Eimeria LDH enzymes was rather low (66-80%), demonstrating an evolutionary divergence. The Plasmodium LDH crystal structure was used to generate a 3D-model structure of E. tenella LDH, which demonstrated that the many variations in the primary amino acid sequences (P. falciparum LDH and E. tenella LDH show only 47% identity) had not resulted in altered 3D-structures. Only a single LDH gene was identified in Eimeria, which was active as a homotetramer. The protein was present at similar levels throughout different parasitic stages (oocysts, sporozoites, schizonts and merozoites), but its corresponding RNA was only observed in the schizont stage, suggesting that its synthesis is restricted to the intracellular stage.