The expression of the calcium-binding protein S100 and its subunits alpha and beta in the intracellular protozoon Eimeria tenella was investigated. For this purpose, anti-S100 whole molecule polyclonal antibody and anti-S100 alpha and anti-S100 alpha monoclonal antibodies were used on paraffin sections collected from the cecum, ileum, colon, rectum, and bursa of Fabricius of chicks experimentally infected with E. tenella. One hundred seventy-five 2- to 3-day-old specific pathogen-free chicks were used. The chicks were divided into experiment and control groups, to include 94 and 81 chicks, respectively. All the chicks in the experimental group received 10,000 spored oocytes in 0.5 ml inoculum in a single dose. The duration of the experiment was limited to 18 days. All 175 chicks were sacrificed by cervical dislocation between hours 1 and 432 following inoculation. The streptavidin-biotin complex method was used to detect the immunoreactivity of the antibodies used. The results revealed a positive immune reaction for the S100 whole molecule and for alpha and beta subunits in the schizonts, microgametes, macrogametes, and oocytes of the E. tenella in the cecum, ileum, colon, rectum, and bursa of Fabricius. These results show that both alpha and beta subunits of the S100 protein existed in all the endogenous phases of E. tenella. These proteins are likely to mediate the parasitic development of E. tenella within host cells.