Turn-over of s- and 1-DC was evaluated using lethally irradiated mice reconstituted with bone marrow cells derived from transgenic mice expressing herpes simplex virus type 1-thymidine kinase in DCs (TK mice) as previously described (24, 39). In this system, only the dividing DC precursors are killed by ganciclovir treatment, allowing the assessment of DC turnover. After a 7-day ganciclovir treatment of control and chimeric mice, s- and 1-DCs were analysed as described in Figure 1. Results are given as the percentage of total LN cell number (mean ± SD of four independent experiments).