Non‐lymphoid cells (NLC) were obtained by thoracic‐duct cnnnulation of mesenteric lymphadenectomized (MLNx) congenitally athymic nude rats. The cells were similar to NLC collected directly from testicular lymph and from the thoracic duct of MLNx euthymic rats, Ia+NLC were characterized by a highly irregular, veiled or dendritic surface. Otherwise the cells were heterogeneous with regard to ultrastructure, non‐specific acid esterase activity, and surface antigens. Roughly a third of the cells had surface Thy‐1 antigen. A negative correlation was found between ihe presence of Thy‐1 and non‐specific acid esterase activity. Compared with macrophages and granulocytes they showed limited phagocytosis of simple test substances. However, Ia+NLC often contained lysosomes and some were occasionally observed in the act of phagocytosing and ingesting other cells, thus demonstrating phagocytic ability. Since Ia+NLC were abundant in peripheral lymph but rare in central lymph, they must he held back by the draining lymph nodes. Their relationship to NLC wilhin lymph nodes i