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Immunotoxicology has developed into an integral regulatory requirement of the toxicological assessment of xenobiotics. Histopathological assessment of lymphoid tissues can provide genuine insight into perturbations of lymphoid cell populations. To facilitate retrospective examination of lymphoid organs should concerns over immunotoxicity be raised, we have endeavored to develop a panel of immunohistochemical techniques to demonstrate T-cells and T-cell subsets in formalin-fixed, paraffin-embedded rat lymphoid tissues. We were successful in developing methods for CD3 and CD8 but failed to arrive at a satisfactory technique for the direct demonstration of CD4 in these tissues. Taking the assumption that the majority of mature T-cells are either CD4+or CD8+, we have combined our methods for CD3 and CD8 in a novel dual-labeling IHC method to simultaneously demonstrate CD3, CD8, and, by implication, CD4 in rat spleen, thymus, lymph node, and Peyer's patch.