T-cell receptors (TcR) recognize short linear peptides (9–15 amino acid long), which have been processed by an ‘antigen-presenting cell’ and complexed to products of the major histocompatibility complex (MHC). Peptides of the appropriate shape and charge are able to bind within the groove of the MHC molecule and it is this complex which is recognized by the TcR. The MHC molecules are highly polymorphic, but each individual will only express one maternal and one paternal allele of HLA Class 1 molecule A, B, and C and HLA Class II DR, DP, and DQ. As a result of TcR specificity and MHC restriction, a clone of T cells will bear a specific receptor which has a very limited repertoire of targets (peptide-MHC complexes). When sufficient numbers of TcR are engaged on a T-cell surface, a cascade of signal transduction events is initiated which results in cellular activation. When investigating the immune response, it may be advantageous to be able to identify which MHC molecules bind a particular peptide well and give rise to a vigorous T-cell response. Such information may be useful in generating effective vaccines. In this chapter, we provide examples of Epstein-Barr virus-transformed lymphoid cell lines and fibroblast cell lines to determine MHC restriction elements.