Chicken monoclonal antibody (mAb), 8C3, which is reactive with a sporozoite antigen of Eimeria acervulina, is a potential therapeutic agent against avian coccidiosis caused by Eimeria spp. However, production of large amounts of 8C3 mAb in cell culture system is labor intensive and not cost- effective. Accordingly, recombinant single chain variable fragment ( ScFv) antibody was constructed by amplification of the VH and VL genes from chicken hybridoma, 8C3 and when expressed in E. coli gave 5 mg l(-1). The expressed protein showed antigen binding activity equivalent to that of the parental mAb. In addition, nucleotide sequence comparison of 8C3 gene to the germline chicken VL genes suggested that the gene conversion with Vk pseudogenes might contribute to the diversi. cation of VL genes in chickens.