A critical requirement for cancer vaccines is that they stimulate CD8 ؉ T cell responses. In this study, we tested the ability of a polyvalent melanoma vaccine to induce CD8 ؉ T cell responses to the melanoma associated antigens MAGE-3 and Melan A/MART-1. Fifteen HLA-A2 ؉ patients with resected malignant melanoma were immunized with the vaccine s.c. every 2-3 weeks. CD8 ؉ T cells in peripheral blood reacting to HLA-A2 restricted epitopes on MAGE-3 (FLWG-PRALV) and Melan A/MART-1/(AAGIGILTV) were quantitated using a filter spot assay at baseline and following 4 immunizations. Vaccine immunization induced CD8 ؉ T cells reacting to one or both of these peptides in 9 of the 15 (60%) patients. These cells were CD8 ؉ and HLA-A2 restricted, as reactivity was abrogated by monoclonal antibodies (MAbs) to CD8 and class I HLA, but not by anti-CD4. All responding patients remained recurrence-free for at least 12 months (median 15 months, range 12 to G21 months), whereas melanoma recurred within 3-5 months in non-responders. The differences in outcome were unrelated to differences in disease severity or overall immunological competence between responders and non-responders. Our results demonstrate directly that MAGE-3 and Melan A/MART-1 can stimulate CD8 ؉ T cell responses in humans, and suggest that these responses are protective and surrogate markers of vaccine efficacy.