The recently reported FACS-based CD107 assay has been used in human HIV and CMV antigen models as well as in the ex vivo analysis of tumor cytolytic T cells in a melanoma model by a single group. The purpose of our study was to validate this assay and to use it in previously untested viral and tumor antigen models. Specifically, we investigated the use of the novel CD107 cytotoxicity assay in the detection of influenza and HER2/neu tumor-specific cytolytic CD8+ T cells. CD8+ T cells from HLA-A2+ healthy donors were stimulated with autologous dendritic cells pulsed with FluM or the HER2/neu peptides, E75 or GP2. These CD8+ T cells were then tested in cytotoxicity assays at varying effector:target (E:T) ratios against T2 targets. Cytotoxicity was measured by detection of CD107a and b on the surface of CD8+ T cells. An E:T of 1:5 was found to optimize the resulting percentage of CD8+CD107+ T cells. E75- and GP2-stimulated CD8+ T cells were then tested in cytotoxicity assays with MCF-7 (HER2/neu +HLA-A2+) and AU565 (HER2/neu +HLA-A2-) tumor cells. Cytotoxicity was measured by both the CD107 assay and the 51Cr release assay. Results of cytotoxicity were then correlated between these two assays. In representative experiments, the CD107 assay identified average specific increases for E75- and GP2-stimulated cells of 4.26 and 3.57%, respectively. These results correlated favorably with cytotoxicity as measured by the traditional 51Cr assay. These findings confirm preliminary reports of the CD107 assay and suggest its usefulness for monitoring cancer trials.