Dendritic cells (DC) are inducers of immune responses par excellence. They also seem responsible for the induction of peripheral T cell tolerance. To investigate these opposite functions of DC, we generated a Cre/LoxP-based system that allows inducible antigen presentation by DC in vivo. This enables us to study the immunogical consequences of antigen presentation by resting versus mature DC without adoptively transferring DC and with physiological numbers of endogenous, naive responder T cells. We found that presentation of LCMV-derived CTL epitopes by resting DC resulted in antigen-specific tolerance, which could not be broken by subsequent infection with LCMV. On the other hand, antigen presentation by activated DC primed endogenous CTL to expand and to develop protective effector function.