CD4 T lymphocytes recognize peptide antigens presented by major histocompatibility complex (MHC) class II molecules, and are central to immune activation and regulation. This pivotal role is evident both in immune responses to pathogens and in the initiation of autoimmunity, although antigen-specific CD4 T cells exist at very low frequencies in patient peripheral blood, frequently in the range of 1: 6,000 to 1: 100,000. To identify and analyze these cells, one must traditionally use functional assays, such as antigen-induced proliferation or cytokine secretion, which infer T cell specificity from semiquantitative or qualitative outcome measures. New techniques using MHC tetramers can now be added to the immunologic toolbox to expand these types of assays, improving sensitivity and quantitation. MHC tetramers are multimeric forms of soluble recombinant HLA molecules associated with specific bound peptide antigens. As illustrated in Figure 1, the recombinant MHC molecules are assembled into multimers to provide multiple ligands for enhanced interaction with antigen-specific T cell receptors (TCRs). These multimers are surrogates for the MHC–peptide ligand that is expressed on the surface of an antigen-presenting cell in a normal immune response. Individual MHC–