An experiment was conducted to study the effect of a mixed Eimeria infection on chicken regulatory T cell (Treg) percentages, T-cell percentages, cell proliferation, and cytokine mRNA amounts. Specific-pathogen-free white leghorn birds were raised in battery cages and were challenged with and without coccidial oocysts at 21 d of age. At 6 d post challenge, birds inoculated with coccidial oocysts shed an average of 1.5 × 104 oocysts/gram of feces. At d 6 post challenge, both CD4+ and CD8+ cell percentages in the cecal tonsil were significantly (P < 0.01) increased in challenged birds compared to the control. At d 11, CD4+ cell percentages in the cecal tonsil were higher in the control birds. At 6 d post challenge there was a 6-fold increase (P = 0.18) in IL-10 mRNA content in the cecal tonsils of infected birds compared to the control. At 11 d post challenge there was a 4-fold increase (P = 0.09) in IL-10 mRNA content in the infected birds’ cecal tonsils. At 11 d post challenge there was half as much (P = 0.10) IFN-γ mRNA content in the spleen of infected birds compared to the control. At 6 d post challenge, the CD25 depleted cecal tonsils of infected birds had increased proliferation (P < 0.05) compared to the control. It could be concluded that a mixed Eimeria infection causes an up-regulation of IL-10, Tregs, and CD4+ cells in the cecal tonsils during the late stages of infection.