This study represents an effort to interrupt the life cycle of Eimeria tenella, the parasite that causes cecal coccidiosis in chickens, with a panel of 12 gamont-specific monoclonal antibodies (Mabs). To facilitate Mab screening, it was first necessary to develop a cell culture-adapted strain of E. tenella (field strain 80) from primary chicken kidney cells (PCKC) to white Leghorn chickens and back to PCKC. This alternation was repeated through 12 such passages. As a result, we have developed a cell culture-adapted strain of E. tenella that produces over 280% more oocysts in vitro than the original parent strain, and over six times as many oocysts as reported by other investigators. Of the 12 Mabs evaluated, Mab HD8 had the greatest inhibitory effect by significantly reducing (P<0.05) the number of E. tenella oocysts produced in cell culture by 61% compared with untreated controls. Western blot analysis revealed that the target antigen for Mab HD8 had a molecular mass of 71 and 36 kilodaltons, respectively, when run under nonreduced and reduced conditions. Subsequent evaluation of soluble and membrane-associated proteins indicated that the target antigen for Mab HD8 was membrane-associated. In vivo studies are now required to validate the efficacy of this antigen in a subunit vaccine.