Dendritic cells are morphologically distinct cells isolated in vitro from peripheral lymphoid organs of mice. They have a buoyant density of less than 1.082 and can be enriched 7–20-fold by isopycnic centrifugation in albumin columns. Surface adherence of enriched populations may yield cultures containing 50% dendritic cells—preparations which can then be studied in more detail. By functional tests, dendritic cells do not represent morphological variants of either lymphocytes or macrophages. They lack lymphocyte surface differentiation markers and do not exhibit the endocytic capacities of macrophages. In tissue culture, they do not differentiate into macrophages. Dendritic cells have a low labeling index in vitro (1.5–2.5%) following administration of [3H]thymidine, and this property distinguishes them from large lymphocytes and promonocytes. Dendritic cells also do not possess the functional properties of other types of reticular cells proposed to exist in lymphoid organs, i.e., they do not synthesize collagen-like macromolecules, they are not stem cells for erythroid and myeloid colony formation, and they do not retain antigens or immune complexes on their cell surface. Dendritic cells thus represent a novel cell type on both functional and morphological grounds.