Polyclonal rabbit antiserum to human T-cell CD3 was used to study its reactivity in lymphoid tissues (lymph nodes, spleen, aggregated lymphoid follicles [Peyer's patches], thymus) of several animal species (cattle, sheep, goats, rats, and mice). Using a peroxidase-antiperoxidase technique on formalin-fixed and paraffin-embedded tissues, immunoreactive cells were detected in T cell-dependent areas of the lymphoid tissues. Reactivity was high in all species tested, but mouse tissues had reduced reactivity, compared with the other species. To obtain a reaction, it was necessary to digest tissues with pronase before application of the immunocytochemical technique. Our results indicate that CD3 antiserum may specifically recognize T-lymphoid cells as it does in human lymphoid tissues and can be used as a marker to study physiologic and pathologic conditions of the lymphoid system of these species.