Thymic epithelial cells (TECs) and dendritic cells are essential for the maintenance of thymopoiesis. Because these stromal elements defi ne the progenitor niche, provide critical survival signals and growth factors, and direct positive and negative selection, detailed study of these populations is necessary to understand important elements for thymic renewal after cytotoxic injury. Study of TEC is currently hindered by lengthy enzymatic separation techniques with decreased viability. We present a new rapid separation technique that yields consistent viable TEC numbers in a quarter of the prior preparation time. Using this new procedure, we identify changes in stromal populations following total body irradiation (TBI). By fl ow cytometry, we show that TBI signifi cantly depletes UEA+ medullary TEC, while sparing Ly51+ CD45− cells. Further characterization of the Ly51+ subset reveals enrichment of fi broblasts (CD45− Ly51+ MHCII−), while cortical TECs (CD45− Ly51+ MHCII+) were markedly reduced. Dendritic cells (CD11c+ CD45+) were also decreased following TBI. These data suggest that cytotoxic preparative regimens may impair thymic renewal by reducing critical populations of cortical and medullary TEC, and that such thymic damage can be assessed by this new rapid separation technique, thereby providing a means of assessing optimal conditioning pretransplant for enhancing thymic-dependent immune reconstitution posttransplant.