The fate of thymic emigrants had so far been studied using a variety of markers, each of which had inherent limitations as to stability, toxicity, or selectivity. We describe a new technique which relies on thein vivoinjection of CFSE, an esterified vital dye hitherto used at 80 times lower concentrations forin vitrocell labeling. We show that CFSE labels a representative sample of all thymocyte subsets and that these migrate at a rate of approximately 2–3 × 10 6 cells/day to peripheral lymphoid organs. We show that they enter lymph nodes at day 1 postinjection and stay for at least 21 days, whereas the turnover in the spleen is more rapid. We also show by immunohistochemistry, using peroxidase-labeled anti-FITC antibodies, that CFSE-labeled thymic emigrants are confined to T-dependent areas of peripheral lymphoid organs.