A new generation of single-cell assays for measuring the frequency of peptide-specific T lymphocytes in cellular populations has become widely available. These assays, enzyme-linked immunospot (ELISPOT), cytokine flow cytometry, and tetramer binding, are used frequently for monitoring of cancer patient responses to vaccination therapies. We concomitantly used these three assays to determine the frequency of CD8(+) T cells in the circulation of 8 patients with metastatic melanoma who had received a multiepitope peptide/dendritic cell-based vaccine. Using peripheral blood mononuclear cells harvested before and a week after the last of four vaccines, we observed that the three assays detected a substantially different frequency of peptide-specific CD8(+) T cells. The tetramer assay consistently detected the highest numbers of peptide-specific CD8(+) T cells, followed by cytokine flow cytometry and then ELISPOT. There was no significant concordance among the assays in measuring the numbers of CD8(+) T cells specific for each of the peptides in the peripheral circulation of the patient. No significant pre- to postvaccine changes in the number of CD8(+) T cells specific for any of the peptides were observed. Thus, the dendritic cell-based vaccine was observed not to augment immune responses to the peptides in the patients. Because of a low frequency of the peptide-specific T cells in the peripheral circulation of the patients, these sensitive single-cell assays were used to measure values at the lower limit of detection. For this and other reasons, including the issues of tetramer specificity and ELISPOT sensitivity, caution in interpretation and serious attention to quality control are needed in monitoring of immune responses to anticancer vaccines.