The tracking of labeled immune cells in living small animals and over time has been a long cherished goal in understanding the mechanism and function of these cells. The advent of fast NMR imaging at near microscopic resolution has provided the opportunity to track cells in deep organs. Unfortunately, few methods exist for high efficiency magnetic cell labeling unless large magnetic particles are bound to the cell surface, which typically has an impact on in vivo homing and biodistribution patterns. The investigators hypothesized that peptide based translocation signals, for example, those derived from the HIV-Tat protein, could be used to shuttle magnetic nanoparticles into cells more efficiently than is possible by fluid phase endocytosis, a method previously developed for magnetic cell tracking. In preliminary studies this group synthesized a number of Tat-derived peptides and determined their capacity to be internalized into cells. In one study an 11-mer Tat sequence was coupled to a magnetic particle and the investigators showed that lymphocyte labeling was improved over 100-fold compared to the non-derivatized preparations. In further studies this group has shown that this powerful new technique for the first time allows 1) tracking of relatively few cells in vivo (< 15 cells per voxel are detectable by MR microscopy); 2) magnetic recovery and isolation of in vivo homed lymphocytes; small amounts. The goals of the current study are: 1) to optimize the newly developed magnetic probes for labeling of lymphocytes; 2) to characterize labeled and unlabeled immune cells; and 3) to apply the developed techniques to questions regarding cytotoxic T-lymphocyte (CTL) distribution in tumors. The investigators believe that these and related questions are highly relevant to tracking immune cells in vivo over time, and in evaluating novel therapies. Furthermore, the investigators believe that the proposed research is in the widest interest to the current NIAID program and other research programs in further developing robust tools to track immune cells in vivo.