Cells with dendritic shape, the so-called dendritic cells (DCs), have been described in many tissues. In order to characterize one DCs population, normal human thymus specimens were obtained from children undergoing cardiovascular surgery. These specimens were either put in culture or fixed for in situ ultrastructural, immunocytochemical and cytochemical studies. In culture, DCs could be differentiated from other non-lymphoid cell populations. They presented long, fine processes and an irregular nucleus. Like interdigitating cells (IDCs) in situ, their cytoplasm contained many free ribosomes and mitochondria, and a well-developed endoplasmic reticulum and Golgi complex. They showed a variable number of tubulovesicular structures and membrane-bound dark homogeneous granules. They never displayed phagolysosomes, tonofilaments or desmosomes. They were Ia+, ATPase+, S-100 protein+, vimentin+, esterase-, lysozyme-, and cytokeratin- cells. Macrophages were easily identified by their numerous lysosomes and large phagolysosomes. They were esterase+, lysozyme+, vimentin+, ATPase +/-, S-100 protein- and cytokeratin-. Although they were Ia+, membrane labelling was not as important as on DC's membrane. In situ, S-100 protein-positive cells had a dendritic shape and were located mainly in medullary regions and at the cortico-medullary border. The staining was diffused both in the nucleus and in the cytoplasm. Lysozyme-positive cells were randomly distributed in the cortex, the medulla and the connective septa. They were round cells and the staining was intracytoplasmic. These observations demonstrate that DCs can be isolated in human thymic cultures, and they suggest that these cells correspond to IDCs in situ. They also provide evidence to suggest that DCs and macrophages are two distinct cellular populations.