In immune intervention trials, the comprehensive investigation of immunogenicity or T-cell epitope-mapping is challenging especially when a large set of epitopes needs to be screened and limited sample material is available. To this end, T-cell responses are often monitored using peptide pools. Here, we assessed the magnitude and sensitivity of detection of antigen-specific CD8(+) and CD4(+) T cells using a single peptide alone or mixed into large pools. Interestingly the magnitude of ex vivo anti-viral and anti-tumor T-cell responses was identical irrespective of the presence and number of irrelevant peptides, in different functional assays with PBMCs from healthy donors and cancer patients. Moreover, the presence of up to 300 irrelevant peptides did not affect the threshold of responsiveness of antigen-specific CD8(+) T cells to single cognate peptides. These data demonstrate the relevance of using very large peptide pools for the sensitive and specific immune-monitoring of epitope-specific T cells in natural or immune-modulated context.