In the present study S-100 protein containing cells in the caecal tonsil were investigated, both at light microscopic and at electron microscopic levels, after oral coccidia inoculation (Eimeria tenella). The birds were infected with a single (Day 0) or two (Days 0 and 21) infective doses of 500 oocysts. Immunoelectron reactivity for S-100 protein was demonstrated in infected chickens, but not in controls. It was found in follicular dendritic cells and interdigitating dendritic cells which were present in the germinal center and diffuse lymphoid tissue respectively, both of them being located in the deep lymphoid tissue near the muscular layer and around the deep glands. Outside the lymphoid tissue, immunoreactivity for S-100 protein was found in cells lying between the epithelial cells of the deep crypt epithelium. Positivity for S-100 protein was observed at 3, 12, 18, 24 and 48 h after the first inoculation as well as on Days 3, 5, 6, 7, 8, 25 and 30 of the experiment. Positive reaction for S-100 protein was detected both while schizont (the more immunogenic stage) development occurs and the number of sporozoites in the caeca lumen was higher, as well as when the production of oocysts reached a maximum. Complementary studies demonstrated that S-100 positive dendritic cells gave a negative reaction for esterase activity, whereas a subset of S-100 negative intraepithelial lymphocytes located between epithelial cells lining the deep glands exhibited esterase activity. These esterase positive cells are hypothesized to be involved in the regulation of local defences.