gp1OO is a melanocytic lineage-specific antigen recognized by tumor-infiltrating lymphocytes, the adoptive transfer of which is associated with tumor regression in melanoma patients. In this study, peripheral blood mononuclear cells (PBMCs) were harvested from HLA-A2+ melanoma patients before and after immunization with G9-209 (ITDQVPFSY), G9-280 (YLEPGPVTA), or G9-154 (KTWGQYWQV) peptides in Incomplete Freund's Adjuvant and were tested for the ability to be sensitized in vitro using PBMCs pulsed with the native peptides. In addition, PBMCs from patients receiving the G9-209 or G9-280 peptide were stimulated in vitro with peptides modified at anchor residues to enhance binding to HLA-A2: G9-209/2M (IMDQVPFSY) or G9-280-9V (YLEPGPVTV). In patients immunized with G9-209, a single in vitro restimulation with G9-209/2M resulted in the generation of specific antipeptide lymphocytes from seven of seven postimmune PBMCs and only three of seven preimmune PBMCs. In patients immunized with G9-280, a single in vitro restimulation with G9-280/9V resulted in the generation of specific antipeptide lymphocytes from five of six postimmune PBMCs and four of six preimmune PBMCs. In almost all cases, CTLs raised against modified epitopes were capable of recognizing targets displaying the native nonamers. Several anti-G9-209 and anti-G9-209/2M CTLs also demonstrated specific lysis of, and specific IFN-gamma release in response to, gp1OO+-established cell lines. Thus, using peptides modified to enhance immunogenicity for in vitro stimulation improved the sensitivity of immune monitoring of patients immunized with synthetic peptides. These results demonstrate that immunization with a peptide derived from a tumor-associated protein such as gp100 can provoke a measurable antitumor immune response in cancer patients.