We have established a sensitive ELISPOT assay measuring interferon g (IFN g) release on a single-cell basis to detect influenza peptide-specific CD8 1 T cells in uncultured peripheral blood mononuclear cells (PBMC). Using this method, we studied the T cell response to HLA-A1 and HLA-A2.1 binding peptide epitopes derived from the MAGE-1 and MAGE-3 proteins, from the melanoma-associated antigens tyrosinase, Melan-A/MART-1 and gp100, and from influenza proteins in stage IV melanoma patients and healthy controls. In 18 of 24 HLA-A2-positive donors (75%), but only in 9 of 25 HLA-A2positive melanoma patients (36%) T cells reactive with the influenza matrix peptide were demonstrated (p 5 0.007). T cells responding to one or several of the melanomaassociated peptides were detected in 5 of 25 HLA-A2-positive patients with metastatic melanoma. Four of these 5 patients had been treated with interleukin-2-and IFNa-containing therapy. Two of the 24 healthy donors had T cells reactive with the MART-1 27-35 peptide. No reactivity with the HLA-A1-binding peptides from MAGE-1 or MAGE-3 was detected in any of the HLA-A1-positive healthy controls or melanoma patients. These results show that the IFNg-ELISPOT assay is suitable to determine quantitatively T cells reactive with melanoma-associated and influenza peptide epitopes in uncultured PBMC. The failure to detect T cells responding to influenza in many melanoma patients with progressive disease may indicate an impairment of their T cell function.