The development of strategies to visualize antigen-specific T-cell responses by flow cytometry has radically changed our understanding of the dynamics and magnitude of virus-induced T-cell immunity. Through the use of tetrameric–major histocompatibility complex (MHC) complexes, massive expansions of antigen-specific CD8+ T cells have been characterized for a variety of pathogens1–5. In addition, this technique has been used for the detection of auto-reactive and tumor-reactive T-cell immunity in vitiligo and melanoma patients6, 7, demonstrating its value for the detection of low frequencies of self-reactive T cells. Here, we describe the use of MHC tetramers for the direct in situ visualization of antigen-specific T-cell responses. The combined use of MHC tetramer technology and confocal laser scanning microscopy (CLSM) allows detection of both virusand tumor-specific CD8+ T cells in lymphoid organs, peripheral tissues and tumor infiltrates.