A simultaneous morphological and quantitative profile was obtained of the cells of blood, thoracic duct, and renal hilar lymph in the dog. Monolayer cytocentrifuged preparations were used to determine the number, type, and size of cells in the three compartments. The cell count of renal lymph was not related to that of blood or thoracic duct lymph. There was a greater percentage of lymphoid cells in the afferent lymph than could be accounted for by the random movement of cells from the blood to the lymph. Thus, there appeared to be a selective transit of cells from blood to lymph. Monocytes and neutrophils were largely absent from the thoracic duct lymph; however, eosinophils were present. Cells were observed in hilar lymph that were characteristic of cells subjected to antigenic stimulation. It was concluded that lymphocytes have a preferential pathway from blood to lymphatic and in the course of this pathway they undergo a change which is consistent with an active immunological role.