The phenotypic analysis of human thymic dentritic cells (DC) in culture and in purified suspensions has been studied with light and electron microscopic (EM) immunolabelling techniques. Using a series of monoclonal antibodies (mAb) and a protein A-gold technique, we demonstrated that DR- and T6-positive cultured DC strongly bind the 9.3F10 mAb, an anti-DR-related antibody produced against human blood DC, and weakly express the T4 antigen, a membrane marker shared by Langerhan's cells (LC). On the other hand, thymic-cultured DC are negative for the other T-cell and monocyte-macrophage antigens. These results support the hypothesis that human thymic DC may be related to blood DC and epidermal LC. Moreover DC, unlike thymic macrophages, do not phagocytose latex particles, opsonized sheep red blood cells (SRBC) or Candida albicans. An efficient two-step technique of isolation, using a Percoll density gradient followed by an indirect panning technique, yields a purified (70-80%) thymic DC population, OKIa1-, 9.3F10- and OKT6-positive and esterase-negative. Immunolabelling and electron microscopy confirm that these isolated DC present similar phenotypic and ultrastructural features to human thymic DC in situ and in culture. Purified DC, used as stimulator cells in mixed leucocyte reaction (MLR), induce stronger proliferative responses than peripheral blood monocytes used as a control; blocking assays with OKIa1 mAb plus complement greatly reduced this stimulatory potency. These functional assays demonstrate that we obtained a purified typical DC population that can be used in immunological functional studies to elucidate the specific role of DC in human thymus.