The biology of antigen presenting cells (APC) traditionally is studied in tissue culture systems using T cells that have been expanded beforehand by stimulation with antigen. Here we consider the distinctive roles of dendritic cells for sensitizing or priming T cells both in vitro and in vivo. Several functions of dendritic cells have been identified in tissue culture that are pertinent to T cell sensitization. These include the ability to a) capture and retain foreign antigens in an immunogenic form, b) bind antigen-specific resting lymphocytes, and c) activate T cells to produce lymphokines and undergo long term clonal growth. Dendritic cells have several properties in vivo that also would contribute to APC function. These are a) their widespread tissue distribution permitting access to antigens in most organs, b) the capacity to home via the blood stream and afferent lymph to the T-dependent areas of spleen and lymph node, and c) the ability to capture antigen in antigen-pulsed animals. Dendritic cells bearing antigen have been administered in situ to initiate responses like contact sensitivity, graft rejection, and antibody formation. A most striking recent example is that, when dendritic cells are pulsed with protein antigens in vitro and administered to immunologically naive mice, there is direct priming of antigen-specific T cells that are restricted to the MHC of the injected APC.