Thoracic-duct cannulation of mesenteric lymphadenectomized (MLNx) congenitally athymic nude rats was studied as a method of obtaining peripheral lymph cells. A higher recovery of non-lymphoid cells (NLC) was obtained from nude than from euthmyic littermates. Both a higher percentage and a greater number of NLC were found in nude animals. Most of these cells resembled dendritic or veiled cells and were strongly positive for Ia antigens. This population could further be enriched by irradiation of the animal, but with a risk of cell damage. Splenectomy had no effect on early output of Ia+ NLC. A substantial population of lymphoid cells from MLNx nude rats expressed T-cell antigens defined by the monoclonal antibodies OX 19, OX 8 or W3/25. These cells were more radiosensitive than were mature T cells. In addition, a large population of cells in the peripheral lymph from nude rats did not display surface antigens recognized by monoclonal antibodies directed either against B cells or against T cells. This cell fraction increased after irradiation. These cells resembled small lymphocytes but had a more irregular nucleus and multiple large granules.