A procedure for analysing the topographical localization in tissue sections or whole-organ mounts of lymphocytes labelled with an intracellular DNA-binding fluorochrome, Hoechst dye No. 33342, is described. The localization of intravenously injected lymphocytes in spleen, popliteal lymph nodes, and Peyer's patches was followed up to 7 days. In the case of spleen, both B and T lymphocytes initially localised in the marginal zone. Subsequently, B cells appeared to exit via the red pulp, while T cells aggregated around vessels in the white pulp. In Peyer's patches, B and T lymphocytes localized to different lymphoid areas. The advantages and potential applications of this technique are discussed.