Histological alterations in the paracortical zones of rabbit lymph nodes were produced by the intravenous injection of the acridine dye euchrysin. The paracortical parenchyma was reduced to narrow bands of lymphoid tissue which cuffed and outlined the venules against a distended and cell-filled sinus system. These perivenous lymphocyte cuffs, termed the paracortical cords, represented the basic anatomical unit of the thymus-dependent paracortical area. Both the paracortical cords and sinuses are cell traffic pathways, but during stages of antigen-induced lymphocyte trapping the cords retain more cells than do the sinuses. Proliferation of T cells and differentiation of B cells both occur within these cords in a microenvironment conducive to cellular cooperation.