Cecal coccidiosis, caused by the protozoan Eimeria tenella, may manifest as a devastating disease in young chickens and result in substantial economic loss for producers. The parasite progresses through a complex life cycle, exhibiting both asexual and sexual (gamont) stages of development. The purpose of this study was to produce a panel of monoclonal antibodies (MoAbs) against epitopes contained on surface antigens (Ags) of the gamonts of E. tenella with the intent of blocking the fertilization process. Gamonts were harvested from infected ceca, partially purified by differential centrifugation throught a discontinuous 5050% Percoll density gradient and used as a source of Ag for the production of MoAbs. Immune spleen cells collected from Robertsonian (strain RBF/Dn) mice were fused with FOX-NY myeloma cells and the resultant MoAb-secreting hybridomas screened by an indirect immunofluorescent antibody test (IFAT). A panel of 13 MoAbs (1 IgG2a and 12 IgG1) was selected form a bank of 94 hybridomas. The Ag specificity of the MoAbs was determined by processing infected cecal mucosa smears and noninfected and infected cecal cross sections through the IFAT procedures. It is likely that the panel of 13 MoAbs exhibits specificity for Ags on or in the macrogamonts of E. tenella. Specificity may not be restricted to macrogamonts, however, since common epitopes make exist between microgamonts and microgamonts. In vitro studies were begun to determine the ability of the MoAbs to inhibit gamont fertilization. Merozoites were inoculated into a monolayer of chick kidney cells, and in vitro development of the parasite was monitored. Data were insufficient for statistical analysis, since the merozoites did not develop to the oocyst stage.