Monoclonal antibodies specific for defined peptide-MHC complexes are now being used to physically detect T-cell receptor ligands. These reagents have resulted in the identification of the cells that present antigen in lymphoid and non-lymphoid tissues after various forms of antigen administration. In addition, recent advances in real-time imaging technology have begun to measure the rate and directionality of T-cell movement relative to antigen-presenting cells in lymph nodes, shedding light on the earliest events in T-cell activation in a physiological setting.