Several properties of lymphoid dendritic cells in situ have been determined, and contrasted to information previously established for lymphocytes and mononuclear phagocytes. Dendritic cells are not found in newborn mice, and their concentration in both spleen and mesenteric lymph node does not reach adult levels until 3–4 wk of age. Dendritic cells largely disappear from adherent populations following administration of steroids (2.5 mg hydrocortisone acetate s.c.) and ionizing radiation (D_o of 100 rads for Co⁶⁰). Splenic dendritic cells can originate from precursors located in both bone marrow and spleen itself, probably the red pulp. The mature splenic population does not actively divide (pulse labeling index with [³H]thymidine of 1.5–2.5%), but does turnover at substantial rate, 10+% of the total pool per day. The influx of new cells appears to be derived from a proliferating precursor compartment, but the mechanism for efflux or turnover is not known. Dendritic cells in spleen and node undergo little or moderate increase in numbers during development of a primary immune response. These in vivo characteristics, taken together, further distinguish dendritic cells as a novel cell type, distinct from mononuclear phagocytes and lymphocytes.