Tracking antigen-specific cytotoxic T lymphocyte (CTL) function in vivo can be difficult due to the need to monitor the presence and subsequent destruction of antigen-bearing target cells. In this report, we describe a simple method using the fluorescent dye 5- (and 6-) carboxyfluorescein diacetate succinimidyl ester (CFSE) to evaluate CD8+ T-cell effector function in vivo by flow cytometry. In this assay, peptide-pulsed and control target cells are labeled to different levels with CFSE and coadministered to animals that have been previously immunized or tolerized to the cognate antigen. Because naive antigen-specific CD8+ T cells cannot acquire effector function within the time frame of this assay, adoptively transferred nonimmunized animals are used as negative controls for in vivo CTL function. Target cells are syngeneic splenocytes pulsed with peptide antigen and control cells are unpulsed syngeneic splenocytes. The loss of antigen-specific target cells is indicative of cytotoxicity and immunity, whereas the lack of killing in the setting of antigen recognition is suggestive of tolerance.