Advances in immunological monitoring provide the means to track tumor antigen-specific cytotoxic T-lymphocytes (CTL) in humans after adoptive transfer with greater specificity and sensitivity than before. Novel tools can be used not only to detect antigen-specific CTL, but also to evaluate the function and phenotype of individual T-cells. Peptide major histocompatibility complexes (MHC) class I multimeric complexes are proving invaluable as fluorescent reagents for tracking of antigen-specific T-cells. The multimeric complex is constructed of four synthetic and biotinylated peptide-loaded MHC molecules, which are linked by a fluorochrome-labeled streptavidin molecule. In contrast to “indirect” assays such as limiting dilution analysis (LDA) or ⁵¹Cr release assays requiring in vitro stimulation, this method allows direct monitoring of very low numbers of peptide-specific T-cells without the need for in vitro sensitization. By combining the use of multimers with anticytokine antibodies, a more detailed picture of the tracked T-cell can be obtained. This chapter summarizes the application of these protocols to monitor ex vivo the frequency and functional activity of tumor-specific CTL after adoptive transfer. Furthermore, it details potential problems interfering with correct staining and provides guidance for interpretation of results.