Technical Abstract: To better understand host cell-parasite interactions during coccidial infection, duodenal tissues were removed from: 1) SC chickens 24 h post primary infection (ppi) with 2x107 oocysts, 2) TK chickens 24,48, and 72 h ppi with 2.5x107 normal or irradiated oocysts, and 3) SC and TK chickens at 24,48 and 72 h post secondary infection with 2.5x107 oocysts. Tissues were imbedded in freezing compound, frozen, sectioned, and stained with monoclonal antibodies to identify CD4+,CD8+,lg+,Bu1a+ lymphocytes, macrophages, and parasites. At 24 h after all infections, sporozoites were seen primarily in CD8+ cells, and macrophages. Large numbers of CD8+ cells were seen in the sections at all times after the infections and were frequently in contact with infected epithelial cells. Meront development at 48 h ppi was extensive with normal oocysts and sparse with irradiated oocysts, but moderate during a secondary infection. These results suggest that sporozoites are transported primarily by CD8+ cells and/or macrophages and that CD8+ cells may mediate host immunity to coccidian parasites.