An enzyme-linked immunospot (ELISPOT) assay was adapted to detect peptide-specific CD8+ T lymphocytes in peripheral blood mononuclear cells (PBMCs). In HLA-A1-, HLA-A2-, and/or HLA-A3-positive individuals, we determined the release of IFN-gamma on a single cell level in response to three different peptide epitopes derived from the influenza matrix protein and nuclear protein containing the HLA-A2.1- and HLA-A1- or HLA-A3-binding motif, respectively. Comparison of the ELISPOT assay with the standard chromium release assay revealed a close correlation between the number of peptide-specific IFN-gamma-releasing T cells in PBMCs and the level of specific cytotoxicity after 14 days of in vitro expansion. The ELISPOT assay detected T cells with specificity for the HLA-A2. 1-binding epitope derived from the matrix protein in 76% of HLA-A2-positive healthy individuals (n = 25); the median frequency was 41 in 10(6) PBMCs. We also detected peptide-specific T cells in 10 of 12 HLA-A2-positive patients with metastatic melanoma with a median frequency of 20.5 in 10(6) PBMCs. In 10 of 24 HLA-A3-positive individuals and in 2 of 14 HLA-A1-positive individuals, peptide-specific T cells for a HLA-A3- and a HLA-A1-binding epitope derived from the nucleoprotein, respectively, were present. In conclusion, the ELISPOT assay may be suitable to monitor a peptide-specific T-cell response in vaccination protocols using peptides derived from tumor or viral antigens.