SUMMARYThe distribution kinetics of i.v. injected51Cr‐labeled CBA lymph node cells were studied in normal CBA mice and in mice subjected to various types of lymphoid depletion. Cells from normal animals were transiently delayed in the lungs and spleens of normal recipients and accumulated gradually in lymph nodes during the first 24 hr. The proportional distribution of labeled lymphocytes was similar over a cell dose range of 105‐108/recipient. The proportion of donor lymph node cells capable of “homing” to lymph nodes of normal recipients was greatly reduced by the following donor treatments: neonatal thymectomy, adult thymectomy, irradiation, syngeneic bone marrow reconstitution, or prolonged drainage of the abdominal lymphatic duct. Exposure of donors to 600 R total body X‐irradiation curtailed migration of donor cells to both lymph node and spleens of recipients, suggesting that more than one population of donor lymphocytes was affected. Donor treatment with repeated closes of heterologous antilymphocyte serum (ALS) profoundly and preferentially affected recirculating lymphocytes, reducing the representation in donor lymph nodes to levels much below those seen after neonatal thymectomy or thoracic duct drainage. The syngeneic51Cr‐labeled cell transfer assay may provide a means of analyzing the action of immunosuppressive agents in terms of their effects on the distribution of lymphoid cell subpopulations.