Peripheral lymph contains constitutively major histocompatibility complex (MHC) class II-positive, dendritic leucocytes (DL) with a veiled (VC) or dendritic (DC) shape. On the basis of phenotypic similarities to the interdigitating cells (IDC) of T-dependent areas of lymphoid tissues, they are believed, but not yet proven, to be IDC precursors that develop into mature IDC within the draining lymph nodes. In order to determine the relationship between VC/DC and IDC, highly purified radiolabelled peripheral lymph DL (collected by thoracic duct cannulation of mesenteric lymphadenectomized congenitally athymic, nude rats) were reinjected into the veins or footpads of syngeneic recipients. Later, host blood and lymph were collected and the tissues were biopsied and processed for scintillation counting or for light microscopic autoradiography combined with electron microscopy of neighbouring sections. The intravenously injected VC/DC did not enter lymph nodes or Peyer's patches directly from blood and did not re-enter the lamina propria of the gut, but became localized largely in the liver and the spleen. From the liver some of the cells proceeded via afferent lymphatics to the liver and the spleen. From the liver some of the cells proceeded via afferent lymphatics to the coeliac lymph nodes. Similarly, some of the locally injected cells were carried with peripheral lymph to the draining popliteal lymph nodes, although most were retained in the footpads. In the spleen the VC/DC entered the periarteriolar lymphoid sheaths and in the coeliac and popliteal lymph nodes they became localized in the paracortex, where they acquired the morphology of mature IDC.