The distribution and evolution of thymic dendritic cells (DC) were studied during human ontogeny. Immunochemical techniques were used to detect S100 protein-positive cells on fetal thymus sections, at different post-fecundation (PF) ages; an image analysis of these positive cells was then carried out. Variations in the percentage of these cells in the medulla were determined according to age: the higher percentages were seen at 12 and 16 weeks PF. Dendritic cells were present at an early stage in the thymic rudiment (7 weeks PF), a finding consistent with an origin of these cells in the fetal liver, and with the possible existence of local attraction and/or maturation factors. The expression of the CD54 adhesion molecule revealed the existence of particular interactions between DC and lymphocytes in the medullary area, and at the cortico-medullar junction. Diffuse CD11a (LFA-1) expression on lymphocytes was consistent throughout gestation. The monocyte/macrophage markers (CD11b, CD14) and the reaction of non-specific esterases showed that the distribution pattern of these cells differed from the DC pattern. These ontogenic data are related to the significant role played by DC throughout the different stages of thymopoiesis.