Kinetics of Lin^- wild-type and IL-6^-/- BM cell proliferation and CFU-GM production on irradiated wild-type and IL-6^-/- stromal layers. Wild-type and IL-6^-/- LTBMCs were established and maintained as in Figure 1. After 4 weeks, cultures were irradiated (17 Gy); 3 days after irradiation, cultures were seeded with 5 × 10⁵ Lin^- wild-type or IL-6^-/- BM cells. No differences were observed in CFU-GM number between Lin^- wild-type and IL-6^-/- BM seeded cells. Half of the medium was changed weekly, and production of total hematopoietic cells (A) and CFU-GMs (B) in culture medium was evaluated. Results are expressed as the mean ± SD of 4 experiments. ^*P < .01, for wild-type stroma versus IL-6^-/- stroma seeded with the same type of BM cells.