A primary infection of 12.5 x 10(3) oocysts of Eimeria adenoeides , given to 20-d-old turkey poults, resulted in depression of weight gain, and the production of large numbers of oocysts in the feces, compared with uninfected controls. Poults were raised under conditions to prevent possible reinfection to determine the ability of the primary infection to confer protective immunity against a challenge infection of 5 x 10(4) oocysts given at 34 d of age. Using weight gain and oocyst production after challenge as criteria for protection, the results indicated that immunity had developed. The concentration and proportions among white blood cell (WBC) populations in peripheral blood were determined at different times after the primary infection. The WBC concentration of infected poults was elevated on d 7 and 11, primarily due to elevated levels of lymphocytes and monocytes on d 7 and eosinophils on d 11. There were no differences in heterophil and basophil concentrations between infected and uninfected poults at any of the time points examined. With the exception of increased percentages of eosinophils on d 11, infection was not associated with alterations in the proportions among WBC populations. Comparison of CD4+ and CD8+ defined lymphocyte subpopulations in the blood of infected versus uninfected poults revealed higher concentrations of CD4+ lymphocytes on d 11, lower concentrations of CD8+ cells on d 4, and higher concentrations of CD8+ cells on d 11 of infection, as well as elevated ratios of CD4+:CD8+ lymphocytes in infected birds on d 4 and 11. These alterations in WBC profiles after primary E. adenoeides infection in turkey poults suggest initiation of both innate and adaptive cellular immune activities designed to effectively cope with a parasitic, intracellular pathogen.