To measure turkey CD4 and CD8 T cell levels, the cross-reactivity of mouse anti-chicken CD4 and CD8 monoclonal antibodies (mAb) with turkey leucocytes was tested by flow cytometric analysis of blood obtained from individuals in five turkey lines. The turkey lines used included a randombred control population (RBC2), a subline (F) of RBC2 selected for increased 16-wk BW, and a sire line (A, B, and C) from each of three commercial turkey breeders. Peripheral blood lymphocytes were isolated and stained with single or dual color staining. The CT8 mAb (anti-chicken CD8alpha) failed to detect the CD8alpha molecule in some turkeys, and there were large line differences in ability to detect the CD8alpha molecule. However, certain anti-chicken CD8alpha mAb (3-298, 3-292, and 11-39) had good cross-reactivity with the turkey CD8alpha molecule. These present data indicate that the turkey CD8alpha molecule is polymorphic. Some anti-chicken CD4 mAb (CT4, 2-6, 2-35, and 7-125) were also cross-reactive with the turkey CD4 molecule. Immunoprecipitation and Western blotting showed that the 3-298 mAb precipitated a 33- to 35-kDa polypeptide from the turkey splenocyte lysate under reducing conditions. The availability of cross-reactive anti-chicken CD4 and CD8 mAb will facilitate the studies of immune responses in turkeys.