Technical Abstract: The chicken monoclonal antibody (mAb) 6D12-G10, raised against Eimeria acervulina sporozoites, has previously been shown to recognize the conoid of E. sporozoites and inhibit sporozoite invasion of lymphocytes in vitro. In this study, we examined the cross-reactivity of mAb 6D12-G10 with other motile stage of Eimeria, sporozoites from 6 different species of Eimeria and 2 other related apicomplexans. In indirect immunofluorescent assay, the mAb 6D12-G10 also reacted with merozoites from E. acervulina and identified a 21kDa merozoite protein in Western blots. By confocal laser scanning microscopy, the conoid of sporozoites from six different avian Eimeria species (E. brunetti, E. maxima, E. mitis, E. necatrix, E. praecox and E. tenella) were reactive with 6D12-G10 mAb. Furthermore, the 6D12-G10 mAb also showed cross-reactivity with motile stages of two closely related apicomplexans including Neospora and Toxoplasma. These results indicate that the mAb 6D12-G10 identifies a conserved epitope on the conoid which is important in host cell invasion by the apicomplexan parasites.